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THE 

DEVELOPMENT  OF  THE  HUMAN  BODY 


McMURRICH 


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MORRIS'S  ANATOMY 

FOURTH  EDITION 
UNDER  AMERICAN  EDITORSHIP 

Rewritten,  Revised,  Improved,  with  Many  New  Illustrations 

EDITED    BY 

HENRY  MORRIS,  F.R.C.S. 

Consulting  Surgeon  to,  and  formerly  Lecturer  on  Surgery  and  Anatomy  at 

Middlesex  Hospital,  London,  and  Examiner  in  Anatomy, 

University  of  Durham,  etc. 


J.  PLAYFAIR  McMURRICH,  A.M.,  Ph.D. 

Professor  of  Anatomy  in  the  University  of  Toronto;  formerly  Professor  of 
Anatomy,  University  of  Michigan 

Among  the  American  contributors  will  be  noted :  J.  Playf air  McMurrich,  R. 
J.  Terry,  Irving  Hardesty,  G.  Carl  Huber,  Abram  T.  Kerr,  Charles  R. 
Bardeen  and  Florence  R.  Sabih.  Henry  Morris,  R.  Marcus  Gunn  and 
W.  H.  A.  Jacobson  head  the  English  contributors. 

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The  text  has  been  completely  revised.  Very  especial  attention,  in  this  new 
edition,  has  been  paid  to  the  illustrations,  with  the  result  that  the  teaching  value 
of  the  book  has  been  very  materially  increased. 

It  contains  many  features  of  special  advantage  to  students.  It  is  modern,  up  to 
date  in  every  respect.  It  has  been  carefully  revised,  and  in  many  parts  rewritten, 
and  includes  many  new  illustrations. 

Containing  about  1024  Illustrations,  of  which  many  are  in  Colors. 
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THE 

DEVELOPMENT  OF  THE 
HUMAN  BODY 

A  MANUAL  OF  HUMAN  EMBRYOLOGY 


BY 

J.  PLAYFAIR  McMURRICH,  A.  M.,  Ph.  D.,  LL.  D. 

PROFESSOR   OF  ANATOMY   IN  THE   UNIVERSITY   OF  TORONTO 
FORMERLY  PROFESSOR   OF   ANATOMY   IN  THE   UNIVERSITY  OF   MICHIGAN 


FOURTH  EDITION,  REVISED  AND  ENLARGED 


With  Two  Hundred  and  Eighty-five  Illustrations  Several 
of  which  are  Printed  in  Colors 


PHILADELPHIA 

P.   BLAKISTON'S   SON   &   CO. 

1012  WALNUT   STREET 
1914 


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(£0\ 


Copyright,  1913,  by  P.  Blakiston's  Son  &  Co. 


THE*  MAPLE.  PRESS-  YORK-  PA 


PREFACE  TO  THE  FOURTH  EDITION, 


The  increasing  interest  in  human  and  mammalian  embryology 
which  has  characterized  the  last  few  years  has  resulted  in  many 
additions  to  our  knowledge  of  these  branches  of  science,  and  has 
necessitated  not  a  few  corrections  of  ideas  formerly  held.  In  this 
fourth  edition  of  this  book  the  attempt  has  been  made  to  incorpo- 
rate the  results  of  all  important  recent  contributions  upon  the  topics 
discussed,  and,  at  the  same  time,  to  avoid  any  considerable  increase 
in  the  bulk  of  the  volume.  Several  chapters  have,  therefore,  been 
almost  entirely  recast,  and  the  subject  matter  has  been  thoroughly 
revised  throughout,  so  that  it  is  hoped  that  the  book  forms  an 
accurate  statement  of  our  present  knowledge  of  the  development 
of  the  human  body. 

To  several  colleagues  the  author  is  indebted  for  valuable  sug- 
gestions, and  in  this  connection  he  desires  especially  to  thank  Dr. 
J.  C.  Watt  for  much  generous  assistance  in  the  revision  of  the  manu- 
script and  for  undertaking  the  correction  of  the  proof-sheets. 

In  addition  to  the  works  mentioned  in  the  preface  to  the  first 
edition  as  of  special  value  to  the  student  of  Embryology,  mention 
should  be  made  of  the  Handbuch  der  vergleichenden  mid  experimen- 
tellen  Entwickhmgslehre  der  Wirbeltiere  edited  by  Professor  Oscar 
Hertwig  and  especially  of  the  Manual  of  Human  Embryology  edited 
by  Professors  F.  Keibel  and  F.  P.  Mall. 
University  of  Toronto. 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/developmentofhumOOmcmu 


PREFACE  TO  THE  FIRST  EDITION. 


The  assimilation  of  the  enormous  mass  of  facts  which  constitute 
what  is  usually  known  as  descriptive  anatomy  has  always  been  a 
difficult  task  for  the  student.  Part  of  the  difficulty  has  been  due  to 
a  lack  of  information  regarding  the  causes  which  have  determined 
the  structure  and  relations  of  the  parts  of  the  body,  for  without  some 
knowledge  of  the  why  things  are  so,  the  facts  of  anatomy  stand  as  so 
many  isolated  items,  while  with  such  knowledge  they  become  bound 
together  to  a  continuous  whole  and  their  study  assumes  the  dignity 
of  a  science. 

The  great  key  to  the  significance  of  the  structure  and  relations 
of  organs  is  their  development,  recognizing  by  that  term  the  historical 
as  well  as  the  individual  development,  and  the  following  pages  con- 
stitute an  attempt  to  present  a  concise  statement  of  the  development 
of  the  human  body  and  a  foundation  for  the  proper  understanding  of 
the  facts  of  anatomy.  Naturally,  the  individual  development  claims 
the  major  share  of  attention,  since  its  processes  are  the  more  immedi- 
ate forces  at  work  in  determining  the  conditions  in  the  adult,  but 
where  the  embryological  record  fails  to  afford  the  required  data, 
whether  from  its  actual  imperfection  or  from  the  incompleteness 
of  our  knowledge  concerning  it,  recourse  has  been  had  to  the  facts  of 
comparative  anatomy  as  affording  indications  of  the  historical  devel- 
opment or  evolution  of  the  parts  under  consideration. 

It  has  not  seemed  feasible  to  include  in  the  book  a  complete  list 
of  the  authorities  consulted  in  its  preparation.  The  short  bibliog- 
raphies appended  to  each  chapter  make  no  pretensions  to  com- 
pleteness, but  are  merely  indications  of  some  of  the  more  important 
works,  especially  those  of  recent  date,  which  consider  the  questions 
discussed.     For  a  very  full  bibliography  of  all  works  treating  of 

vii 


Vlll  PREFACE    TO    THE    FIRST    EDITION 

human  embryology  up  to  1893  reference  may  be  made  to  Minot's 
Bibliography  of  Vertebrate  Embryology,  published  in  the  "Memoirs 
of  the  Boston  Society  of  Natural  History,"  volume  iv,  1893.  It  is 
fitting,  however,  to  acknowledge  an  especial  indebtedness,  shared 
by  all  writers  on  human  embryology,  to  the  classic  papers  of  His, 
chief  among  which  is  his  Anatomie  menschlicher  Embryonen,  and 
grateful  acknowledgments  are  also  due  to  the  admirable  text-books 

of  Minot,  O.  Hertwig,  and  Kollmann. 

Anatomical  Laboratory, 
University  of  Michigan. 


CONTENTS 

Page 

Introduction 1 

PART  I.— GENERAL  DEVELOPMENT. 

CHAPTER  I. 

The  Spermatozoon  and  Spermatogenesis;  the  Ovum  and  Its  Matu- 
ration and  Fertilization 1 1 

CHAPTER  II. 

The  Segmentation  of  the  Ovum  and  the  Formation  of  the  Germ 

Layers 3& 

CHAPTER  III. 

The  Medullary  Groove,  Notochord,  and  Mesodermic  Somites   ...     64 

CHAPTER  IV. 
The  Development  of  the  External  Form  of  the  Human  Embryo ...      86 

CHAPTER  V. 

The  Yolk-stalk,  Belly-stalk,  and  Fetal  Membranes 107 

PART  II.— ORGANOGENY. 

CHAPTER  VI. 
The  Development  of  the  Integumentary  System 141 

CHAPTER  VII. 

The  Development  of  the  Connective  Tissues  and  Skeleton    .    .    .        153 

CHAPTER  VIII. 

The  Development  of  the  Muscular  System 193 

ix 


X  CONTENTS 

Page 

CHAPTER  IX. 

The  Development  of  the  Circulatory  and  Lymphatic  Systems   ..    .    .    221 

CHAPTER  X. 
The  Development  of  the  Digestive  Tract  and  Glands 280 

CHAPTER  XL 

The  Development  of  the  Pericardium,  the  Pleuro-peritoneum,  and 

the  Diaphragm 316 

CHAPTER  XII. 
The  Development  of  the  Organs  of  Respiration 331 

CHAPTER  XIII. 
The  Development  of  the  Urinogenital  System 338 

CHAPTER  XIV. 
The  Suprarenal  System  of  Organs      370 

CHAPTER  XV. 
The  Development  of  the  Nervous  System 377 

CHAPTER  XVI. 
The  Development  of  the  Organs  of  Special  Sense 427 

CHAPTER  XVII. 

Post-natal  Development 47° 

Index 487 


THE  DEVELOPMENT 

OF  THE 

HUMAN  BODY. 


INTRODUCTION. 

Somewhat  more  than  seventy  years  ago  (1839)  one  of  the  funda- 
mental principles  of  biology  was  established  by  Schleiden  and 
Schwann  as  the  cell  theory.  According  to  this,  all  organisms  are 
composed  of  one  or  more  structural  units  termed  cells,  each  of  which, 
in  multicellular  organisms,  maintains  an  individual  existence  and 
yet  contributes  with  its  fellows  to  the  general  existence  of  the  indi- 
vidual. Viewed  in  the  light  of  this  theory,  the  human  body  is  a 
community,  an  aggregate  of  many  individual  units,  each  of  which 
leads  to  a  certain  extent  an  independent  existence  and  yet  both 
contributes  to  and  shares  in  the  general  welfare  of  the  community. 

To  the  founders  of  the  theory  the  structural  units  were  vesicles 
with  definite  walls,  and  little  attention  was  paid  to  their  contents. 
Hence  the  use  of  the  term  "cell"  in  connection  with  them.  Long 
before  the  establishment  of  the  cell  theory,  however,  the  existence 
of  organisms  composed  of  a  gelatinous  substance  showing  no  indica- 
tions of  a  definite  limiting  membrane  had  been  noted,  and  in  1835  a 
French  naturalist,  Dujardin,  had  described  the  gelatinous  material 
of  which  certain  marine  organisms  (Rhizopoda)  were  composed, 
terming  it  sarcode  and  maintaining  it  to  be  the  material  substratum 
which  conditioned  the  various  vital  phenomena  exhibited  by  the 
organisms.  Later,  in  1846,  a  botanist,  von  Mohl,  observed  that 
living  plant  cells  contained  a  similar  substance,  upon  which  he 


2  INTRODUCTION 

believed  the  existence  of  the  cell  as  a  vital  structure  was  dependent, 
and  he  bestowed  upon  this  substance  the  name  protoplasm,  by  which 
it  is  now  universally  known. 

By  these  discoveries  the  importance  originally  attributed  to  the 
cell-wall  was  greatly  lessened,  and  in  1864  Max  Schultze  reformu- 
lated the  cell  theory,  defining  the  cell  as  a  mass  of  protoplasm,  the 
presence  or  absence  of  a  limiting  membrane  or  cell-wall  being 
immaterial.  At  the  same  time  the  spontaneous  origination  of  cells 
from  an  undifferentiated  matrix,  believed  to  occur  by  the  older 
authors,  was  shown  to  have  no  existence,  every  cell  originating  by 
the  division  of  a  preexisting  cell,  a  fact  concisely  expressed  in  the 
aphorism  of  Virchow — omnis  cellula  a  cellula. 

Interpreted  in  the  light  of  these  results,  the  human  body  is  an 
aggregate  of  myriads  of  cells,* — i.  e.,  of  masses  of  protoplasm,  each 
of  which  owes  its  origin  to  the  division  of  a  preexistent  cell  and  all  of 
which  may  be  traced  back  to  a  single  parent  cell— a  fertilized  ovum. 
All  these  cells  are  not  alike,  however,  but  just  as  in  a  social  community 
one  group  of  individuals  devotes  itself  to  the  performance  of  one  of 
the  duties  requisite  to  the  well-being  of  the  community  and  another 
group  devotes  itself  to  the  performance  of  another  duty,  so  too, 
in  the  body,  one  group  of  cells  takes  upon  itself  one  special 
function  and  another  another.  There  is,  in  other  words,  in 
the  cell-community  a  physiological  division  of  labor.  Indeed, 
the  comparison  of  the  cell-community  to  the  social  community  may 
be  carried  still  further,  for  just  as  gradations  of  individuality  may  be 
recognized  in  the  individual,  the  municipality,  and  the  state,  so  too 
in  the  cell-community  there  are  cells;  tissues,  each  of  which  is  an 
aggregate  of  similar  cells;  organs,  which  are  aggregates  of  tissues,  one, 
however,  predominating  and  determining  the  character  of  the  organ; 
and  systems,  which  are  aggregates  of  organs  having  correlated 
functions. 

It  is  the  province  of  embryology  to  study  the  mode  of  division  of 

*  It  has  been  estimated  that  the  number  of  cells  entering  into  the  composition  of 
the  body  of  an  adult  human  being  is  about  twenty-six  million  five  hundred  thousand 
millions! 


INTRODUCTION  3 

the  fertilized  ovum  and  the  progressive  differentiation  of  the  resulting 
cells  to  form  the  tissues,  organs,  and  systems.  But  before  consider- 
ing these  phenomena  as  seen  in  the  human  body  it  will  be  well  to  get 
some  general  idea  of  the  structure  of  an  animal  cell. 

This  (Fig.  i),  as  has  been  already  stated,  is  a  mass  of  protoplasm, 
a  substance  which  in  the  living  condition  is  a  viscous  fluid  resembling 
in  many  of  its  peculiarities  egg-albumen,  and  like  this  being  coagu- 
lated when  heated  or  when  exposed  to 
the  action  of  various  chemical  reagents. 
As  to  the  structure  of  living  protoplasm 
little  is  yet  known,  since  the  application 
of  the  reagents  necessary  for  its  accurate 
study  and  analysis  results  in  its  disin- 
tegration or  coagulation.  But  even  in 
the  living  cell  it  can  be  seen  that  the  Fig.  i.— Ovum  of  New-born 
protoplasm  is  not  a  simple  homogeneous  ?^ILr?  WInTH  Follicle-cells~ 
substance.     What  is  termed  a  nucleus  is 

usually  clearly  discernible  as  a  more  or  less  spherical  body  of  a 
greater  refractive  index  than  the  surrounding  protoplasm,  and  since 
this  is  a  permanent  organ  of  the  .cell  it  is  convenient  to  distinguish 
the  surrounding  protoplasm  as  the  cytoplasm  from  the  nuclear 
protoplasm  or  karyoplasm. 

The  study  of  protoplasm  coagulated  by  reagents  seems  to  indi- 
cate that  it  is  a  mixture  of  substances  rather  than  a  simple  chemical 
compound.  Both  the  cytoplasm  and  the  karyoplasm  consist  of  a 
more  solid  substance,  the  reticulum,  which  forms  a  network  or  felt- 
work,  in  the  interstices  of  which  is  a  more  fluid  material,  the  enchy- 
lema*  The  karyoplasm,  in  addition,  has  scattered  along  the  fibers 
of  its  reticulum  a  peculiar  material  termed  chromatin  and  usually 
contains  embedded  in  its  substance  one  or  more  spherical  bodies 

*  It  has  been  observed  that  certain  coagulable  substances  and  gelatin,  when  sub- 
jected to  the  reagents  usually  employed  for  "fixing"  protoplasm,  present  a  structure 
similar  to  that  of  protoplasm,  and  it  has  been  held  that  protoplasm  in  the  uncoagulated 
condition  is,  like  these  substances,  a  more  or  less  homogeneous  material.  On  the 
other  hand,  Biitschli  maintains  that  living  protoplasm  has  a  foam-structure  and  is, 
in  other  words,  an  emulsion. 


4  INTRODUCTION 

termed  nucleoli,  which  may  be  simply  larger  masses  of  chromatin  or 
bodies  of  special  chemical  composition.  And,  finally,  in  all  actively 
growing  cells  there  is  differentiated  in  the  cytoplasm  a  peculiar  body 
known  as  the  archo plasm  sphere,  in  the  center  of  which  there  is 
usually  a  minute  spherical  body  known  as  the  centrosome. 

It  has  been  already  stated  that  new  cells  arise  by  the  division  of 
preexisting  ones,  and  this  process  is  associated  with  a  series  of  com- 
plicated phenomena  which  have  great  significance  in  connection  with 
some  of  the  problems  of  embryology.  When  such  a  cell  as  has  been 
described  above  is  about  to  divide,  the  fibers  of  the  reticulum  in 
the  neighborhood  of  the  archoplasm  sphere  arrange  themselves  so  as 
to  form  fibrils  radiating  in  all  directions  from  the  sphere  as  a  center, 
and  the  archoplasm  with  its  contained  centrosome  gradually  elon- 
gates and  finally  divides,  each  portion  retaining  its  share  of  the  radiat- 
ing fibrils,  so  that  two  asters,  as  the  aggregate  of  centrosome,  sphere 
and  fibrils  is  termed,  are  now  to  be  found  in  the  cytoplasm  (Fig.  2,  A) . 
Gradually  the  two  asters  separate  from  one  another  and  eventually 
come  to  rest  at  opposite  sides  of  the  nucleus  (Fig.  2,  C).  In  this 
structure  important  changes  have  been  taking  place  in  the  mean- 
time. The  chromatin,  originally  scattered  irregularly  along  the 
reticulum,  has  gradually  aggregated  to  form  a  continuous  thread 
(Fig.  2,  A),  and  later  this  thread  breaks  up  into  a  definite  number 
of  pieces  termed  chromosomes  (Fig.  2,  B),  the  number  of  these  being 
practically  constant  for  each  species  of  animal.  In  man  the  number 
has  been  placed  at  twenty-four  (Flemming,  Duesberg) ,  but  the  recent 
observations  of  Guyer  indicate  that  it  is  probably  twenty-four  in  the 
female  and  twenty-two  in  the  male.  The  significance  of  this  differ- 
ence in  the  two  sexes  will  be  considered  in  connection  with  the 
fertilization  of  the  ovum  (p.  32). 

As  soon  as  the  asters  have  taken  up  their  position  on  opposite 
sides  of  the  nucleus,  the  nuclear  reticulum  begins  to  be  converted 
into  a  spindle-shaped  bundle  of  fibrils  which  associate  themselves 
with  the  astral  rays  and  have  lying  scattered  among  them  the  chro- 
mosomes (Fig.  2,  C).  To  the  figure  so  formed  the  term  amphiaster  is 
applied,  and  soon  after  its  formation  the  chromosomes  arrange 


INTRODUCTION 


5 


themselves  in  a  circle  or  plane  at  the  equator  of  the  spindle  (Fig.  2,  D) 
and  the  stages  preparatory  to  the  actual  division,  the  prophases,  are 
completed. 

The  next  stage,  the  metaphase  (Fig.  3,  A),  consists  of  the  division, 
usually  longitudinally,  of  each  chromosome,  so  that  the  cell  now 


Fig.  2. — Diagrams  Illustrating  the  Prophases  of  Mitosis. — {Adapted  from 

E.  B.  Wilson.) 


contains  twice  as  many  chromosomes  as  it  did  previously.  As  soon 
as  this  division  is  completed  the  anaphases  are  inaugurated  by  the 
halves  of  each  chromosome  separating  from  one  another  and  ap- 
proaching one  of  the  asters  (Fig.  3,  B),  and  a  group  of  chromosomes, 
containing  half  the  total  number  formed  in  the  metaphase,  comes  to 


6  INTRODUCTION 

lie  in  close  proximity  to  each  archoplasm  sphere  (Fig.  3,  C).  The 
spindle  and  astral  fibers  gradually  resolve  themselves  again  into  the 
reticulum  and  the  chromosomes  of  each  group  become  irregular  in 
shape  and  gradually  spread  out  upon  the  nuclear  reticulum  so  that 
•two  nuclei,  each  similar  to  the  one  from  which  the  process  started, 


Fig.  3. — Diagrams  Illustrating  the  Metaphase  and  Anaphases  of  Mitosis.: — 
{Adapted  from  E.  B.  Wilson.) 

are  formed  (Fig.  3,  D).  Before  all  these  changes  are  accomplished, 
however,  a  constriction  makes  its  appearance  at  the  surface  of  the 
cytoplasm  (Fig.  3,  C)  and,  gradually  deepening,  divides  the  cyto- 
plasm in  a  plane  passing  through  the  equator  of  the  amphiaster  and 
gives  rise  to  two  separate  cells  (Fig.  3,  D). 


INTRODUCTION  7 

This  complicated  process,  which  is  known  as  karyokinesis  or 
mitosis,  is  the  one  usually  observed  in  dividing  cells,  but  occasionally 
a  cell  divides  by  the  nucleus  becoming  constricted  and  dividing  into 
two  parts  without  any  development  of  chromosomes,  spindle,  etc., 
the  division  of  the  cell  following  that  of  the  nucleus.  This  ami- 
totic method  of  division  is,  however,  rare,  and  in  many  cases,  though 
not  always,  its  occurrence  seems  to  be  associated  with  an  impairment 
of  the  reproductive  activities  of  the  cells.  In  actively  reproducing 
cells  the  mitotic  method  of  division  may  be  regarded  as  the  rule. 

Since  the  process  of  development  consists  of  the  multiplication  of 
a  single  original  cell  and  the  differentiation  of  the  cell  aggregate  so 
formed,  it  follows  that  the  starting-point  of  each  line  of  individual 
development  is  to  be  found  in  a  cell  which  forms  part  of  an  individual 
of  the  preceding  generation.  In  other  words,  each  individual 
represents  one  generation  in  esse  and  the  succeeding  generation  in 
posse.  This  idea  may  perhaps  be  made  clear  by  the  following  con- 
siderations. As  a  result  of  the  division  of  a  fertilized  ovum  there  is 
produced  an  aggregate  of  cells,  which,  by  the  physiological  division  of 
labor,  specialize  themselves  for  various  functions.  Some  assume 
the  duty  of  perpetuating  the  species  and  are  known  as  the  sexual 
or  germ  cells,  while  the  remaining  ones  divide  among  themselves  the 
various  functions  necessary  for  the  maintenance  of  the  individual, 
and  may  be  termed  the  somatic  cells.  The  germ  cells  represent 
potentially  the  next  generation,  while  the  somatic  cells  constitute  the 
present  one.     The   idea   may  be  represented   schematically  thus: 

First  generation 

Somatic  cells  +  germ  cells 

II 
Second  generation 

Somatic  cells  +  germ  cells 

II 
Third  generation 


Somatic  cells  +  germ  cells,  etc. 

It  is  evident,  then,  while  the  somatic  cells  of  each  generation  die 
at  their  appointed  time  and  are  differentiated  anew  for  each  genera- 


8  INTRODUCTION 

tion  from  the  germ  cells,  the  latter,  which  may  be  termed  collectively 
the  germ-plasm,  are  handed  on  from  generation  to  generation  without 
interruption,  and  it  may  be  supposed  that  this  has  been  the  case  ab 
initio.  This  is  the  doctrine  of  the  continuity  of  the  germ-plasm,  a 
doctrine  of  fundamental  importance  on  account  of  its  bearings  on 
the  phenomena  of  heredity. 

It  is  necessary,  however,  to  fix  upon  some  link  in  the  continuous 
chain  of  the  germ-plasm  as  the  starting-point  of  the  development 
of  each  individual,  and  this  link  is  the  fertilized  ovum.  By  this  is 
meant  a  germ  cell  produced  by  the  fusion  of  two  units  of  the  germ- 
plasm.  In  many  of  the  lower  forms  of  life  (e.g.,  Hydra  and  certain 
turbellarian  worms)  reproduction  may  be  accomplished  by  a  division 
of  the  entire  organism  into  two  parts  or  by  the  separation  of  a  portion 
of  the  body  from  the  parent  individual.  Such  a  method  of  repro- 
duction is  termed  non-sexual.  Furthermore  in  a  number  of  forms 
(e.  g.,  bees,  Phylloxera,  water-fleas)  the  germ  cells  are  able  to  undergo 
development  without  previously  being  fertilized,  this  constituting 
a  method  of  reproduction  known  as  parthenogenesis.  But  in  all 
these  cases  sexual  reproduction  also  occurs,  and  in  all  the  more  highly 
organized  animals  it  is  the  only  method  that  normally  occurs;  in  it  a 
germ  cell  develops  only  after  complete  fusion  with  another  germ  cell. 
In  the  simpler  forms  of  this  process  little  difference  exists  between 
the  two  combining  cells,  but  since  it  is,  as  a  rule,  of  advantage  that 
a  certain  amount  of  nutrition  should  be  stored  up  in  the  germ  cells 
for  the  support  of  the  developing  embryo  until  it  is  able  to  secure  food 
for  itself,  while  at  the  same  time  it  is  also  advantageous  that  the  cells 
which  unite  shall  come  from  different  individuals  (cross-fertilization), 
and  hence  that  the  cells  should  retain  their  motility,  a  division  of 
labor  has  resulted.  Certain  germ  cells  store  up  more  or  less  food 
yolk,  their  motility  becoming  thereby  impaired,  and  form  what  are 
termed  the  female  cells  or  ova,  while  otners  discard  all  pretensions  of 
storing  up  nutrition,  are  especially  motile  and  can  seek  and  pene- 
trate the  inert  ova;  these  latter  cells  constitute  the  male  cells  or 
spermatozoa.  In  many  animals  both  kinds  of  cells  are  produced  by 
the  same  individual,  but  in  all  the  vertebrates  (with  rare  exceptions 


INTRODUCTION  9 

in  some  of  the  lower  orders)  each  individual  produces  only  ova  or 
spermatozoa,  or,  as  it  is  generally  stated,  the  sexes  are  distinct. 
It  is  of  importance,  then,  that  the  peculiarities  of  the  two  forms 
of  germ  cells,  as  they  occur  in  the  human  species,  should  be  con- 
sidered. 

LITERATURE. 

E.  B.  Wilson:  "The  Cell  in  Development  and  Inheritance."     Third  edition.     New 

York,  1900. 
O.  Hertwig:  "Die  Zelle  und  die  Gewebe."     Jena,  1893. 


PART   I. 

GENERAL  DEVELOPMENT. 


CHAPTER  I. 


THE    SPERMATOZOON    AND    SPERMATOGENESIS;     THE 

OVUM  AND  ITS  MATURATION  AND 

FERTILIZATION. 

The  Spermatozoon. — The  human  spermatozoon  (Figs.  4  and  5) 
is  a  minute  and  greatly  elongated  cell,  measuring  about  0.05  mm.  in 
length.  It  consists  of  an  anterior  broader  portion  or  head  (Fig.  5,  H) , 
which  measures  about  0.005  mm-  in  length  and,  when  viewed  from 
one  surface  (Fig.  4,  1),  has  an  oval  outline,  though  since  it  is  some- 
what flattened  or  concave  toward  the  tip,  it  has  a  pyriform  shape 
when  seen  in  profile  (Fig.  4,  2).  Covering  the  flattened  portion  of 
the  head  and  fitting  closely  to  it  is  a  delicate  cap-like  membrane, 
the  head-cap  (Fig.  5,  He),  whose  apex  is  a  sharp  edge,  this  structure 
corresponding  to  a  pointed  prolongation  of  the  cap  found  in  the 
spermatozoon  of  many  of  the  lower  vertebrates  and  known  as  the 
perforatorium.  Immediately  behind  the  head  is  a  short  portion 
known  as  the  neck  (Fig.  5,  N),  which  consists  of  an  upper  more 
refractive  body,  the  anterior  nodule,  and  a  lower  clearer  portion. 
To  this  succeeds  the  connecting  or  middle-piece  (Figs.  4  and  5,  m) 
which  begins  with  a  posterior  nodule,  from  the  center  of  which  there 
passes  back  through  the  axis  of  the  piece  an  axial  filament,  enclosed 
within  a  sheath,  this  latter  having  wrapped  around  it  a  spiral  fila- 
ment. At  the  lower  end  of  the  middle-piece  this  spiral  filament 
terminates  in  the  annulus,  through  which  the  axial  filament  and  its 
sheath  passes  into  the  jiagellum  or  tail  (Fig.  4,/).     This  portion, 


12 


THE    SRERMATOZOON 


which  constitutes  about  four-fifths  of  the  total  length  of  the  sper- 
matozoon is  composed  simply  of  the  axial  filament  and  its  sheath, 
this  latter  gradually  thinning  out  as  it  passes  backward  and  ceasing 
altogether  a  short  distance  above   the  end  of  the  axial   filament. 


H.  { 


N. 


M. 


Fig.  4. — Human  Spermatozoon. 
1,  Front  view;  2,  side  view  of  the 
head;  e,  terminal  filament;  k,  head; 
/,   tail;    m,   middle-piece. — (After 
Retzius.) 


Fig.  5. — Diagram  Showing  the  Structure 
of  a  Human  Spermatozoon. 

Af,  Axial  filament;  Ann,  annulus;  H,  head; 
He,  lower  border  of  head -cap;  m,  middle- piece; 
N,  neck;  Na  and  Np,  anterior  and  posterior 
nodule;  S,  sheath  of  axial  filament;  Spf,  spiral 
filament. — (Bonnet,  after  Meves.) 


The  filament  thus  projects  somewhat  beyond  the  actual  end  of  the 
tail,  forming  what  is  known  as  the  terminal  filament  or  end-piece 
(Fig.  4,  e). 

To  understand  the  significance  of  the  Various  parts  entering  into 
the  composition  of  the  spermatozoon  a  study  of  their  development 
is  necessary,  and  since  the  various  processes  of  spermatogenesis  have 
been  much  more  accurately  observed  in  such  mammalia  as  the  rat 


SPERMATOGENESIS 


13 


and  guinea-pig  than  in  man,  the  description  which  follows  will  be 
based  on  what  has  been  described  as  occurring  in  these  forms. 
From  what  is  known  of  the  spermatogenesis  in  man  it  seems  certain 
that  it  closely  resembles  that  of  these  mammals  so  far  as  its  essential 
features  are  concerned. 

Spermatogenesis. — The  spermatozoa  are  developed  from  the 
cells  which  line  the  interior  of  the  seminiferous  tubules  of  the  testis. 
The  various  stages  of  development  cannot  all  be  seen  at  any  one 
part  of  a  tubule,  but  the  formation  of  the  spermatozoa  seems  to  pass 


Fig.  6. — Diagram  showing  Stages  of  Spermatogenesis  as  seen  in  Different 
Sectors  of  a  Seminiferous  Tubule  of  a  Rat. 
s,  Sertoli  cell;  scl,  spermatocyte  of  the  first  order;  sc2,  spermatocyte  of  the  second 
order;    sg,    spermatogone;    sp,    spermatid;    sz,    spermatozoon. — (Modified  from   von 
Lenhossek.) 


along  each  tubule  in  a  wave-like  manner  and  the  appearances  pre- 
sented at  different  points  of  the  wave  may  be  represented  diagram- 
matically  as  in  Fig.  6. 

In  the  first  section  of  this  figure  four  different  generations  of 
cells  are  represented;  above  are  mature  spermatozoa  lying  in  the 
lumen  of  the  tubule,  while  next  the  basement  membrane  is  a  series 
of  cells  from  which  a  new  generation  of  spermatozoa  is  about  to 
develop.     The  cells  of  this  series  are  of  two  kinds;  the  larger  one  (s) 


14  SPERMATOGENESIS 

will  develop  into  a  structure  known  as  a  Sertoli  cell,  while  the  others 
are  parent  cells  of  spermatozoa  and  are  termed  spermatogonia  (sg). 
In  the  next  section  the  Sertoli  cell  is  seen  to  have  become  consider- 
ably enlarged,  its  cytoplasm  projecting  toward  the  lumen  of  the  tubule, 
and  in  the  third  section  the  enlargement  has  increased  to  such  an 
extent  that  the  spermatogonia  are  forced  away  from  the  basement 
membrane,  with  which  the  Sertoli  cell  alone  is  in  contact.  In  the 
fourth  section  ("he  spermatogonia  are  seen  in  process  of  division; 
one  of  the  cells  so  formed  will  persist  as  a  spermatogone,  while  the 
other  forms  what  is  termed  a  primary  spermatocyte  (sc1).  The 
results  of  the  division  are  seen  in  the  last  section,  where  four  sper- 
matogonia are  seen  again  in  contact  with  the  basement  membrane 
and  above  them  are  four  primary  spermatocytes.  Returning  now 
to  the  first  and  second  sections,  the  layer  of  primary  spermatocytes 
may  still  be  seen,  indications  of  an  approaching  division  being 
furnished  by  the  arrangement  of  the  chromatin  in  those  of  the 
second  section,  and  in  the  third  section  the  division  is  seen  in  prog- 
ress, the  two  cells  which  result  from  it  being  termed  secondary 
spermatocytes  (sc2).  These  cells  almost  immediately  undergo 
division,  as  shown  in  the  fourth  section,  each  giving  rise  to  two 
spermatids  (sp),  each  of  which  becomes  later  on  directly  trans- 
formed into  a  spermatozoon  (sz).  From  each  primary  spermatocyte 
there  have  been  formed,  therefore,  as  the  result  of  two  mitoses,  four 
cells,  each  of  which  represents  a  spermatozoon. 

During  these  divisions  important  departures  from  the  typical 
method  of  mitosis  occur,  these  departures  leading  to  a  reduction  of 
the  chromosomes  in  each  spermatid  to  one-half  the  number  occurring 
in  the  somatic  cells.  The  general  plan  by  which  this  is  accomplished 
may  be  described  as  follows:  In  the  division  of  the  spermatogonia 
the  number  of  chromosomes  that  appears  is  identical  with  that  found 
in  the  somatic  cells,  so  that  in  a  form  whose  somatic  number  is  eight, 
eight  chromosomes  appear  in  each  spermatogonium,  and  divide  so 
that  eight  pass  to  each  of  the  resulting  primary  spermatocytes. 
When  these  cells  divide,  however,  the  number  of  chromosomes  that 
appears  is  only  one-half  the  somatic  number,  namely,  four  in  the 


SPERMATOGENESIS 


15 


supposed  case  that  is  being  described  (Fig.  7,  sc1).  The  further 
history  of  these  chromosomes  indicates  that  each  is  composed  of 
four  elements  more  or  less  closely  united  to  form  a  tetrad,  and  during 
mitosis  each  tetrad  divides  into  two  dyads,  four  of  which  will  there- 
fore pass  into  each  secondary  spermatocyte.     These  cells  (Fig.  7,  sc2) 


Fig.  7. — Diagram  Illustrating  the  Reduction  of  the  Chromosomes  During 

Spermatogenesis. 
sc1,  Spermatocyte  of  the  first  order;  sc2,  spermatocyte  of  the  second  order;  sp, 

spermatid. 


undergo  division  without  the  usual  reconstruction  of  the  nucleus  and 
each  of  the  dyads  which  they  contain  is  halved,  so  that  each  sper- 
matid receives  a  number  of  single  chromosomes  equal  to  half  the 
number  characteristic  for  the  species  (Fig.  7,  sp). 

This  account  of  the  behavior  of  the  chromosomes  during  sper- 


1 6  SPERMATOGENESIS 

matogenesis  assumes  that  all  the  chromosomes  of  the  primary 
spermatocytes  are  of  equal  value  and  behave  similarly  during 
mitosis.  It  has  been  found,  however,  that  in  a  number  of  forms 
(insects,  spiders,  birds,  etc.,)  this  is  not  the  case  and  recent  obser- 
vations by  Guyer  indicate  that  in  man  certain  of  the  spermatocytic 
chromosomes  differ  decidedly  from  their  fellows.  At  the  division 
of  the  primary  spermatocytes  twelve  chromosomes  make  their 
appearance,  but  two  of  these  differ  from  the  rest  in  that  they  do  not 
divide,  but  pass  directly  to  one  of  the  poles  of  the  mitotic  spindle 
(Fig.  8).  When  the  division  is  completed,  accordingly,  one  of  the 
two  daughter  secondary  spermatocytes  will  have  received  two 
undivided  or  accessory  chromosomes  plus  ten  ordinary  chromosomes, 
resulting  from  the  division  of  ten  of  the  primary  spermatocytic 
chromosomes;  the  other  daughter  cell,  on  the  other  hand,  will  have 
received  only  ten  ordinary  chromosomes  in  all,  so  that  two  classes  of 
secondary  spermatocytes  are  formed,  in  one  of  which  the  cells 
possess  twelve  chromosomes  and  in  the  other  only  ten. 

In  this  respect,  then,  the  spermatogenesis  in  man  differs  from  the 
general  plan  described  above  and  the  division  of  the  secondary 
spermatocytes  reveals  a  second  difference.  For  in  these  mitoses 
instead  of  twelve  and  ten  chromosomes,  seven  and  five,  respectively, 
make  their  appearance.  This  may  be  explained  on  the  supposition 
that  the  ten  ordinary  chromosomes,  present  in  each  class  of  secondary 
spermatocytes,  have  united  to  form  five  bivalent  chromosomes, 
while  the  two  accessory  chromosomes,  present  in  one  of  the  classes 
have  remained  distinct.  During  the  mitosis  the  accessory  chromo- 
somes divide  just  as  do  the  ordinary  ones,  so  that  from  each  sperma- 
tocyte of  one  class  two  spermatids  are  formed,  each  containing  seven 
chromosomes,  while  from  each  spermatocyte  of  the  other  class  two 
spermatids,  each  containing  five  chromosomes,  result  (Fig.  8). 
Since  the  spermatids  are  directly  transformed  into  spermatozoa, 
half  of  these  latter  will  have  received  seven  chromosomes,  and  the 
remaining  half  will  have  received  five,  or,  since  the  five  ordinary 
chromosomes  are  bivalent  and  the  two  accessories  are  univalent,  the 
spermatozoa  of  one  class  will  each  have  received  the  equivalent  of 


SPERMATOGENESIS  1 7 

ten  plus  two,  i.  e.,  twelve  univalent  chromosomes,  while  those  of 
the  other  class  will  have  received  the  equivalent  of  only  ten.* 

The  transformation  of  the  spermatids  into  spermatozoa  takes 
place  while  they  are  in  intimate  association  with  the  Sertoli  cells, 
a  number  of  them  fusing  with  the  cytoplasm  of  an  enlarged  Sertoli 
cell,  as  shown  in  Fig.  6,  s,  and  probably  receiving  nutrition  from  it. 
In  each  spermatid  there  is  present  in  addition  to  the  nucleus,  an 


Fig.  8. — Diagram  Illustrating  the  Behavior  of  the  Chromosomes  in  Human 

Spermatogenesis. 
The  upper  figure  shows  the  mitotic  spindle  of  a  primary  spermatocyte  with  the  two 
accessory  chromosomes  passing  to  one  pole.  The  two  figures  in  the  second  row  repre- 
sent the  chromosomes  of  such  a  spindle  in  an  anaphase;  seen  from  either  pole,  and  the 
figures  of  the  last  row  represent  spermatids  derived  from  the  two  classes  of  secondary 
spermatocytes. — (Based  on  Guyer.) 

archoplasm  sphere  and  two  centrosom.es  that  have  migrated  from 
the  archoplasm  and  lie  free  in  the  cytoplasm.  The  centrosomes 
and  the  archoplasm  sphere  take  up  their  position  at  opposite  poles 
of  the  nucleus,  the  archoplasm  eventually  forming  the  head-cap  of  the 
spermatozoon,  and  from  one  of  the  centrosomes  a  slender  axial 

*  Doubt  has  been  thrown  upon  the  accuracy  of  these  observations  by  Gutherz,  who, 
while  he  finds  a  structure  in  the  human  spermatocyte  which  he  identifies  as  an  accessor)' 
chromosome,  claims  that  it  divides  similarly  to  the  other  chromosomes.  He  does  not 
find,  therefore,  any  numerical  difference  in  the  chromosomes  of  the  spermatids  dividing 
them  into  two  classes,  although  there  may  be  qualitative  differences  indistinguishable 
by  our  present  technique. 


15  SPEEMATOGENESIS 

filament  grows  out  and  soon  projects  beyond  the  limits  of  the  cyto- 
plasm (Fig.  g,  A).  The  other  centrosome  becomes  a  rod-shaped 
structure  which  applies  itself  closely  to  the  posterior  pole  of  the 
nucleus,  becoming  the  anterior  nodule,  while  the  lower  one,  from 
which  the  filament  arises,  becomes  at  first  pyramidal  in  shape 
(Fig.  9,  B)  and  later  separates  into  a  rod-like  portion  to  which  the 
filament  is  attached  and  a  ring,  through  which  the  filament  passes 
(Fig.  9,  C).     The  rod-like  portion  becomes  the  posterior  nodule, 


ABC 

Fig.  g. — Stages  in  the  Transformation  of  a  Spermatid  into  a 
Spermatozoon. — (After  Meves.) 


and  the  ring  separates  from  it  to  form  the  annulus  (Fig.  g,D).  The 
nucleus  becomes  the  head  of  the  spermatozoon,  the  cytoplasm  sur- 
rounding it  becoming  reduced  to  an  exceedingly  delicate  layer,  so 
that  the  head  is  composed  almost  entirely  of  nuclear  substance,  if 
the  head-cap  be  left  out  of  consideration.  The  spiral  filament  of 
the  middle-piece  is,  however,  a  derivative  of  the  cytoplasm  and 
according  to  some  authors  this  portion  of  the  spermatid  also  fur- 
nishes the  material  for  the  sheath  of  the  axial  filament,  though 
this  has  been  denied  (Meves),  the  sheath  being  regarded  as  a  differ- 
entiation of  the  axial  filament.  Each  spermatozoon  is,  then,  one 
of  four  equivalent  cells,  produced  by  two  successive  divisions  of  a 
primary  spermatocyte  and  containing  one-half  the  number  of  chromo- 
somes characteristic  for  the  species. 


THE    OVUM  19 

The  number  of  spermatozoa  produced  during  the  lifetime  of  a 
single  individual  is  very  large.  It  has  been  found  that  1  cu.  mm.  of 
human  ejaculate  contains  60,876  spermatozoa,  a  single  ejaculate, 
therefore,  containing  over  200,000,000.  This  would  indicate  that 
during  his  lifetime  a  man  may  produce  340  billion  spermatozoa 
(Lode). 

The  Ovum. — The  human  ovum  is  a  spherical  cell  measuring 
about  0.2  mm.  in  diameter  and  is  contained  within  a  cavity  situated 


-dp 

.0 


mgr — — % 


Fig.  10. — Section  through  Portion  of  an  Ovary  of  an  Opossum  {Didephys  vir- 

giniana)  showing  Ova  and  Follicles  in  Various  Stages  of  Development. 
b,  Blood-vessel;   dp,   discus  proligerus;  mg,  stratum  granulosum;  o,  ovum;  s,  stroma; 

th,  theca  folliculi. 

near  or  at  the  surface  of  the  ovary  and  termed  a  Graafian  follicle. 
This  follicle  is  surrounded  by  a  capsule  composed  of  two  layers,  an 
outer  one,  the  theca  externa,  consisting  of  fibrous  tissue  resembling 
that  found  in  the  ovarian  stroma,  and  an  inner  one,  the  theca  interna, 
composed  of  numerous   spherical   and   fusiform  cells.     Both  the 


20  THE    OVUM 

thecse  are  richly  supplied  with  blood-vessels,  the  theca  interna 
especially  being  the  seat  of  a  very  rich  capillary  network.  Internal 
to  the  theca  interna  there  is  a  transparent,  thin,  and  structureless 
hyaline  membrane,  within  which  is  the  follicle  proper,  whose  wall  is 
formed  by  a  layer  of  cells  termed  the  stratum  granulosum  (Fig.  10,  mg) 
and  inclosing  a  cavity  filled  with  an  albuminous  fluid,  the  liquor 

,^  -  ■'•  —  ■:  '■':.):-  i--.  ■.,■■-■'■'  -    -.-€>)    /    ZP 


V  1 


Fig.  ii. — Ovum  from  Ovary  of  a  Woman  Thirty  Years  of  Age. 

cr,  Corona  radiata;  n,  nucleus;  p,  protoplasmic  zone  of  ovum;  ps,  perivitelline  space; 

y,  yolk;  zp,  zona  pellucida. — (Nagel.) 

folliculi.  At  one  point,  usually  on  the  surface  nearest  the  center 
of  the  ovary,  the  stratum  granulosum  is  greatly  thickened  to  form  a 
mass  of  cells,  the  discus  proligerus  {dp),  which  projects  into  the 
cavity  of  the  follicle  and  encloses  the  ovum  (0) .  Usually  but  a  single 
ovum  is  contained  in  any  discus,  though  occasionally  two  or  even 
three  may  occur. 


OVULATION   AND    THE    CORPUS    LUTEUM  21 

The  cells  of  the  discus  proligerus  are  for  the  most  part  more  or 
less  spherical  or  ovoid  in  shape  and  are  arranged  irregularly.  In 
the  immediate  vicinity  of  the  ovum,  however,  they  are  more  columnar 
in  form  and  are  arranged  in  about  two  concentric  rows,  thus  giving 
a  somewhat  radiated  appearance  to  this  portion  of  the  discus,  which 
is  termed  the  corona  radiata  (Fig.  u,  cr).  Immediately  within  the 
corona  is  a  transparent  membrane,  the  zona  pellucida  (Fig.  n,  zp), 
about  as  thick  as  one  of  the  cell  rows  of  the  corona  (0.02  to  0.024  mm.) , 
and  presenting  a  very  fine  radial  striation  which  has  been  held  to  be 
due  to  minute  pores  traversing  the  membrane  and  containing  delicate 
prolongations  of  the  cells  of  the  corona  radiata.  Within  the  zona 
pellucida  is  the  ovum  proper,  whose  cytoplasm  is  more  or  less  clearly 
differentiated  into  an  outer  more  purely  protoplasmic  portion 
(Fig.  n,  p)  and  an  inner  mass  (y)  which  contains  numerous  fine 
granules  of  fatty  and  albuminous  natures.  These  granules  represent 
the  food  yolk  or  deutoplasm,  which  is  usually  much  more  abundant 
in  the  ova  of  other  mammals  and  forms  a  mass  of  relatively  enormous 
size  in  the  ova  of  birds  and  reptiles.  The  nucleus  (n)  is  situated 
somewhat  excentrically  in  the  deutoplasmic  portion  of  the  ovum  and 
contains  a  single,  well-defined  nucleolus. 

A  follicle  with  the  structure  described  above  and  containing  a 
fully  grown  ovum  may  measure  anywhere  from  five  to  twelve  milli- 
meters in  diameter,  and  is  said  to  be  "mature,"  having  reached  its 
full  development  and  being  ready  to  burst  and  set  free  the  ovum. 
This,  however,  is  not  yet  mature;  it  is  not  ready  for  fertilization,  but 
must  first  undergo  certain  changes  similar  to  those  through  which 
the  spermatocyte  passes,  the  so-called  ovum  at  this  stage  being  more 
properly  a  primary  oocyte.  But  before  describing  the  phenomena  of 
maturation  of  the  ovum  it  will  be  well  to  consider  the  extrusion  of 
the  ovum  and  the  changes  which  the  follicle  subsequently  undergoes. 

Ovulation  and  the  Corpus  Luteum.— As  a  rule,  but  a  single 
follicle  near  maturity  is  found  in  either  the  one  or  the  other  ovary 
at  any  given  time.  In  the  early  stages  of  its  development  a  follicle 
is  situated  somewhat  deeply  in  the  stroma  of  the  ovary,  but  during 
its  growth  it  approaches  the  surface  and  eventually  forms  a  marked 


22 


OVULATION  AND    THE    CORPUS    LUTEUM 


prominence,  only  an  exceedingly  thin  membrane  separating  the 
cavity  of  the  follicle  from  the  abdominal  cavity.  This  thin  mem- 
brane finally  ruptures,  and  the  liquor  folliculi,  which  is  apparently 
under  some  pressure  while  contained  within  the  follicle,  rushes  out 
through  the  rupture,  carrying  with  it  the  ovum  surrounded  by  some 
of  the  cells  of  the  discus  proligerus. 

The  immediate  cause  of  the  bursting  of  the  follicle  is  not  yet 
clearly  understood.  It  has  been  suggested  that  a  gradual  increase 
of  the  liquor  folliculi  under  pressure  must  in  itself  finally  lead  to  a 
rupture,  and  it  has  also  been  pointed  out  that  just  before  the  matura- 
tion of  the  follicle  the  theca  interna  undergoes  an  exceedingly  rapid 
development  and  vascularization  which  may  play  an  important  part 
in  the  phenomenon. 

Normally  the  ovum  when  expelled  from  its  follicle  is  received  at 
once  into  the  Fallopian  tube,  and  so  makes  its  way  to  the  uterus,  in 

whose  cavity  it  undergoes  its  de- 
velopment. Occasionally,  how- 
ever, this  normal  course  may  be 
interfered  with,  the  ovum  coming 
to  rest  in  the  tube  and  there 
undergoing  its  development  and 
producing  a  tubal  pregnancy; 
or,  again,  the  ovum  may  not  find 
its  way  into  the  Fallopian  tube, 
but  may  fall  from  the  follicle 
into  the  abdominal  cavity, 
where,  if  it  has  been  fertilized, 
it  will  undergo  development, 
producing  an  abdominal  preg- 
nancy; and,  finally,  and  still  more  rarely,  the  ovum  may  not  be 
expelled  when  the  Graafian  follicle  ruptures  and  yet  may  be 
fertilized  and  undergo  its  development  within  the  follicle,  bringing 
about  what  is  termed  an  ovarian  pregnancy.  All  these  varieties 
of  extra-uterine  pregnancy  are,  of  course,  exceedingly  serious,  since 
in  none  of  them  is  the  fetus  viable. 


Fig.  12. — Ovary  of  a  Woman  Nine- 
teen Years  of  Age,  Eight  Days  after 
Menstruation. 

d,  Blood-clot;  /,  Graaffian  follicle;  th, 
theca. — (Kollmann.) 


OVULATION  AND    THE    CORPUS    LUTEUM 


23 


With  the  setting  free  of  the  ovum  the  usefulness  of  the  Graafian 
follicle  is  at  an  end,  and  it  begins  at  once  to  undergo  retrogressive 
changes  which  result  primarily  in  the  formation  of  a  structure 
known  as  the  corpus  luteiim  (Fig.  12).     On  the  rupture  of  the  follicle 


Fig.  13. — Section  through  the  Corpus  Luteum  of  a  Rabbit,  Seventy  Hours 

post  coitum. 
The  cavity  of  the  follicle  is  almost  completely  filled  with  lutein  cells  among  which 
is  a  certain  amount  of  connective  tissue,     g,  Blood-vessels;  ke,  ovarial  epithelium. — 
(Sobotta.) 

a  considerable  portion  of  the  stratum  granulosum  remains  in  place, 
and  usually  there  is  an  effusion  of  a  greater  or  less  amount  of  blood 
from  the  vessels  of  the  theca  interna  into  the  follicular  cavity.  The 
split  in  the  wall  of  the  follicle  through  which  the  ovum  escaped  soon 
closes  over  and  the  cavity  becomes  filled  with  cells  separated  into 
groups  by  trabecular  of  connective  tissue  containing  blood-vessels 
(Fig.  13).     These  cells  contain  a  considerable  amount  of  a  peculiar 


24  OVULATION  AND    THE    CORPUS    LUTEUM 

yellow  pigment  known  as  lutein,  the  color  imparted  to  the  follicle 
by  this  substance  having  suggested  the  name  corpus  luteum  which 
is  now  applied  to  it. 

In  later  stages  there  is  a  gradual  increase  in  the  amount  of  con- 
nective tissue  present  and  a  corresponding  diminution  of  the  lutein 
cells,  the  corpus  luteum  gradually  losing  its  yellow  color  and  be- 
coming converted  into  a  whitish,  fibrous,  scar-like  body,  the  corpus 
albicans,  which  may  eventually  almost  completely  disappear.  These 
various  changes  occur  in  every  ruptured  follicle,  whether  or  not  the 
ovum  which  was  contained  in  it  be  fertilized.  But  the  rapidity 
with  which  the  various  stages  of  retrogression  ensue  differs  greatly 
according  to  whether  pregnancy  occurs  or  not,  and  it  is  customary 
to  distinguish  the  corpora  lutea  which  are  associated  with  pregnancy 
as  corpora  lutea  vera  from  those  whose  ova  fail  to  be  fertilized  and 
which  form  corpora  lutea  spuria.  In  the  latter  the  retrogression  of 
the  follicle  is  completed  usually  in  about  five  or  six  weeks,  while  the 
corpora  vera  persist  throughout  the  entire  duration  of  the  pregnancy 
and  complete  their  retrogression  after  the  birth  of  the  child. 

Two  very  different  views  are  held  as  to  the  origin  of  the  lutein 
cells.  According  to  one,  which  may  be  termed  von  Baer's  view, 
the  cells  of  the  stratum  granulosum  remaining  in  the  follicle  rapidly 
undergo  degeneration  and  completely  disappear,  and  the  lutein  cells 
and  connective-tissue  trabecular  are  formed  entirely  from  the  cells  of 
the  theca  interna,  which  increase  rapidly  both  in  size  and  number. 
The  other  view  was  first  advanced  by  Bischoff  and  may  be  known 
by  his  name.  It  is  to  the  effect  that  the  granulosa  cells  do  not  dis- 
integrate, but,  on  the  contrary,  increase  rapidly  in  number  and  be- 
come converted  into  the  lutein  cells,  only  the  connective  tissue  and 
the  blood-vessels  being  derived  from  the  theca  interna. 

Which  of  these  two  views  is  correct  is  at  present  uncertain. 
The  majority  of  those  who  have  within  recent  years  studied  the 
formation  of  the  human  corpus  luteum  have  expressed  themselves 
in  favor  of  von  Baer's  theory.  Sobotta  has,  however,  made  a 
thorough  study  of  the  phenomena  in  a  perfect  series  of  mice  ovaries 
and  has  demonstrated  that  in  that  form  the  lutein  cells  are  derived 


OVULATION  AND    THE    CORPUS    LUTEUM  25 

from  the  granulosa  cells.  It  would  be  strange  if  the  lutein  cells  had 
a  different  origin  in  two  different  mammals,  and  the  observations  on 
mice  are  so  thorough  that  one  is  tempted  to  regard  different  results 
as  being  due  to  imperfections  in  the  series  of  ovaries  studied, 
important  steps  in  the  development  of  the  corpora  lutea  being  thus 
overlooked.  This  temptation  is,  moreover,  greatly  increased  by  the 
fact  that  Sobotta's  observations  have  been  confirmed  in  the  cases  of 
several  other  animals,  such,  for  instance,  as  the  rabbit  (Sobotta, 
Honore,  Cohn),  certain  bats  (van  der  Stricht),  the  sheep  (Marshall), 
the  marsupial  dasyurus  (Sandes),  the  spermophile  (Volker),  and 
the  guinea-pig  (Sobotta).  The  weight  of  evidence  is  at  the  present 
time  strongly  in  favor  of  Bischoff's  view,  but  until  the  adverse 
results  obtained  by  Clarke  and  others  from  the  study  of  the  human 
corpus  luteum  and  those  obtained  by  Jankowski  fiom  the  pig  have 
been  shown  to  be  incorrect,  the  question  as  to  the  invariable  deriva- 
tion of  the  lutein  cells  from  the  stratum  granulosum  must  be  left 
open.  Since  it  is  held  that  both  the  granulosa  and  theca  cells  are 
derivatives  of  the  embryonic  ovarial  epithelium  the  essential  differ- 
ences between  the  two  origins  that  have  been  ascribed  to  the  lutein 
cells  may  not  be  so  great  as  has  been  supposed.  Indeed,  it  is  possible 
that  both  the  follicular  and  thecal  cells  may  in  some  cases  con- 
tribute to  the  formation  of  the  corpus  luteum. 

The  persistence  of  the  corpus  luteum  throughout  the  entire 
period  of  pregnancy  and  its  disappearance  within  a  few  weeks  if 
pregnancy  does  not  supervene,  have  suggested  the  probability  of  its 
being  related  to  the  changes  that  take  place  in  the  uterus  in  con- 
nection with  the  implantation  of  the  ovum  in  its  wall.  Experimental 
removal  of  the  corpus  luteum  in  rabbits  either  before  or  shortly 
after  the  implantation  of  the  ovum  produces  a  failure  of  pregnancy 
(Fraenkel),  and  similar  results  have  been  obtained  in  mice  and 
bitches  (Marshall  and  Jolly).  It  has  accordingly  been  held  that 
the  corpus  luteum  is  an  organ  of  internal  secretion  directly  con- 
cerned in  the  production  and  maintenance  of  the  modifications  of 
the  uterus  necessary  for  the  implantation  and  further  development 
of  the  ovum. 


26  THE    RELATION    OF    OVULATION   TO    MENSTRUATION 

The  Relation  of  Ovulation  to  Menstruation. — It  was  long 
believed  that  ovulation  was  coincident  with  certain  periodic  changes 
of  the  uterus  which  constitute  what  is  termed  menstruation.  This 
phenomenon  makes  its  appearance  at  the  time  of  puberty,  the  exact 
age  at  which  it  appears  being  determined  by  individual  and  racial 
peculiarities  and  by  climate  and  other  factors,  and  after  it  has  once 
appeared  it  normally  recurs  at  definite  intervals  more  or  less  closely 
corresponding  with  lunar  months  ii.  e.,  at  intervals  of  about  twenty- 
eight  days)  until  somewhere  in  the  neighborhood  of  the  fortieth  or 
forty-fifth  year,  when  it  ceases. 

In  each  menstrual  cycle  four  stages  may  be  recognized,  one  of 
which,  the  intermenstrual,  greatly  exceeds  the  others  in  its  duration, 
occupying  about  one-half  the  entire  period.  During  this  stage  the 
mucous  membrane  of  the  uterus  is  practically  at  rest,  but  toward 
its  close  the  membrane  gradually  begins  to  thicken  and  the  second 
stage,  the  premenstrual  stage,  then  supervenes.  This  lasts  for  six  or 
seven  days  and  is  characterized  by  a .  marked  proliferation  and 
swelling  of  the  uterine  mucosa,  the  subjacent  tissue  becoming  at 
the  same  time  highly  vascular  and  eventually  congested.  The 
walls  of  the  blood-vessels  situated  beneath  the  mucosa  then  degen- 
erate and  permit  the  escape  of  blood  here  and  there  beneath  the 
mucous  membrane,  this  leading  to  the  third,  or  menstrual,  stage  in 
which  the  mucous  membrane  diminishes  in  thickness,  those  portions 
of  it  that  overlie  the  effused  blood  undergoing  fatty  degeneration 
and  desquamation,  so  that  the  stage  is  characterized  by  more  or 
less  extensive  hsemorrhage.  The  duration  of  this  stage  is  from 
three  to  five  days  and  then  ensues  the  postmenstrual  stage,  lasting 
from  four  to  six  days,  during  which  the  mucous  membrane  is  re- 
generated and  again  returns  to  the  intermenstrual  condition. 

It  seems  but  natural  to  regard  these  changes  as  the  expression 
of  a  periodic  attempt  to  prepare  the  uterus  for  the  reception  of  the 
fertilized  ovum,  this  preparation  being  completed  during  the 
premenstrual  stage,  the  succeeding  menstrual  and  postmenstrual 
being  merely  the  return  of  the  uterine  mucosa  to  the  resting  inter- 
menstrual stage,  pregnancy  not  having  occurred.     If  this  be  the 


THE    RELATION    OF    OVULATION    TO    MENSTRUATION  27 

real  significance  of  the  menstrual  cycle,  one  would  expect  to  find 
ovulation  occurring  at  a  more  or  less  definite  portion  of  the  cycle, 
at  such  a  time  that  the  ovum,  if  fertilized  would  be  able  to  make 
use  of  the  premenstrual  preparation  for  its  reception. 

Attempts  to  determine  the  relation  of  ovulation  to  menstruation 
have  been  made  by  estimating  the  age  of  the  corpora  lutea  occurring 
in  ovaries  removed  in  the  course  of  operation  from  patients,  the  date 
of  whose  last  menstruation  was  known.  The  results  obtained  by 
this  method  have,  however,  proved  somewhat  discordant.  Thus, 
Fraenkel  records  out  of  eighty-five  cases  ten  in  which  the  operation 
was  performed  immediately  before  or  after  menstruation,  and  in 
none  of  these  was  any  corpus  luteum  present;  further,  in  twenty 
cases  a  newly  formed  corpus  luteum  was  found  and  in  these  cases 
the  last  menstruation  had  occurred  on  the  average  nineteen  (13-27) 
days  previously.  Villemin,  too,  reached  a  similar  result,  concluding 
that  ovulation  took  place  about  fifteen  days  after  menstruation. 
On  the  other  hand,  Leopold  and  Ravano  found  that  in  ninety-five 
cases  ovulation  coincided  with  menstruation  in  fifty-nine,  while  in 
the  remaining  thirty-six  it  occurred  during  other  stages  of  the  cycle. 

If  any  conclusion  may  be  drawn  from  these  contradictory  results 
it  would  seem  to  be  that  in  the  human  species  ovulation  may  take 
place  at  any  stage  of  the  menstrual  cycle.  Indeed,  it  may  also  be 
said  that  ovulation  may  take  place  independently  of  the  menstrual 
cycle,  since  cases  are  on  record  of  pregnancy  having  occurred  in 
girls  who  had  not  yet  menstruated.  In  other  words,  it  seems 
probable  that  ovulation  does  not  depend  upon  the  condition  of  the 
uterine  mucous  membrane,  but  upon  some  other  factor  as  yet 
undetermined. 

'  The  conditions  in  lower  animals  seem  also  to  point  in  this  direction. 
In  these  ovulation  is,  as  a  rule,  associated  with  a  certain  condition  known 
as  oestrus  or  "heat,"  this  being  preceded  by  certain  phenomena  con- 
stituting what  is  termed  the  procestrum  and  corresponding  essentially  to 
menstruation.  In  several  forms,  such  as  the  dog  and  the  pig,  ovulation 
appears  to  occur  regularly  in  association  with  "heat,"  but  in  others,  such 
as  the  cat,  the  mouse  and  probably  the  rabbit,  it  occurs  at  this  time  only 
if   copulation    also   occurs.     Furthermore,  it  has   been   observed   that 


28  THE    MATURATION    OF    THE    OVUM 

although  female  monkeys  menstruate  regularly  throughout  the  year, 
nevertheless  there  is  but  one  annual  cestral  period  when  ovulation  takes 
place  (Heape). 

The  Maturation  of  the  Ovum. — Returning  now  to  the  ovum, 
it  has  been  shown  that  at  the  time  of  its  extrusion  from  the  Graafian 
follicle  it  is  not  equivalent  to  a  spermatozoon  but  to  a  primary 
spermatocyte,  and  it  may  be  remembered  that  such  a  spermatocyte 


Fig.  14. — Ovum  of  a  Mouse  Showing  the  Maturation  Spindle. 

The  ovum  is  enclosed  by  the  zona  pellucida  (z.p),  to  which  the  cells  of  the  corona  radiata 

are  still  attached. — (Sobotta.) 

becomes  converted  into  a  spermatozoon  only  after  it  has  undergone 
two  divisions,  during  which  there  is  a  reduction  of  the  number  of  the 
chromosomes  to  practically  one-half  the  number  characteristic  for 
the  species. 

Similar  divisions  and  a  similar  reduction  of  the  chromosomes 
occur  in  the  case  of  the  ovum,  constituting  what  is  termed  its 
maturation.     The  phenomena  have  not  as  yet  been  observed  in 


THE    MATURATION    OF    THE    OVUM 


20 


human  ova,  and,  indeed,  among  mammals  only  with  any  approach 
to  completeness  in  comparatively  few  forms  (rat,  mouse,  guinea- 
pig,  bat  and  cat);  but  they  have  been  observed  in  so  many  other 
forms,  both  vertebrate  and  invertebrate,  and  present  in  all  cases  so 


Fig.  15. — Diagram  Illustrating  the  Reduction  of~the  Chromosomes  during 

the  Maturation  of  the  Ovum. 
0,  Ovum;  ocl,  oocyte  of  the  first  generation;  oc2,  oocyte  of  the  second  generation; 

p,  polar  globule. 

much  uniformity  in  their  general  features,  that  there  can  be  little 
question  as  to  their  occurrence  in  the  human  ovum. 

In  typical  cases  the  ovum  (the  primary  oocyte)  undergoes  a 
division  in  the  prophases  of  which  the  chromatin  aggregates  to  form 
half  as  many  tetrads  as  there  are  chromosomes  in  the  somatic  cells 


30  THE   MATURATION    OF    THE    OVUM 

(Fig.  15,  oc1)  and  at  the  metaphase  a  dyad  from  each  tetrad  passes 
into  each  of  the  two  cells  that  are  formed.  These  two  cells  (second- 
ary oocytes)  are  not,  however,  of  the  same  size;  one  of  them  is 
almost  as  large  as  the  original  primary  oocyte  and  continues  to  be 
called  an  ovum  (oc2),  while  the  other  is  very  small  and  is  termed  a 
polar  globule  (ft).  A  second  division  of  the  ovum  quickly  succeeds 
the  first  (Fig.  15,  oc2),  and  each  dyad  gives  a  single  chromosome  to 
each  of  the  two  cells  which  result,  so  that  each  of  these  cells  possesses 
half  the  number  of  chromosomes  characteristic  for  the  species. 
The  second  division,  like  the  first,  is  unequal,  one  of  the  cells  being 
relatively  very  large  and  constituting  the  mature  ovum,  while  the 
other  is  small  and  is  the  second  polar  globule.  Frequently  the  first 
polar  globule  divides  during  the  formation  of  the  second  one,  a 
reduction  of  its  dyads  to  single  chromosomes  taking  place,  so  that 
as  the  final  result  of  the  maturation  four  cells  are  formed  (Fig.  15), 
the  mature  ovum  (o),and  three  polar  globules  (ft),  each  of  which 
contains  half  the  number  of  chromosomes  characteristic  for  the 
species. 

The  similarity  of  the  maturation  phenomena  to  those  of  sper- 
matogenesis may  be  perceived  trom  the  following  diagram: 

n/"~N  Spermato- 

(       J  cyte  I 


Spermato- 
cyte II 


Ovum 


Oocyte  II         O  O  OO 


OO  OO  Spermatids 


Polar  globules 


In  both  processes  the  number  of  cells  produced  is  the  same  and  in 
both  there  is  a  similar  reduction  of  the  chromosomes.  But  while 
each  of  the  four  spermatids  is  functional,  the  three  polar  globules 
are  non-functional,  and  are  to  be  regarded  as  abortive  ova,  formed 


THE    FERTILIZATION    OF    THE    OVUM  3 1 

during  the  process  of  reduction  of  the  chromosomes  only  to  undergo 
degeneration.  In  other  words,  three  out  of  every  four  potential 
ova  sacrifice  themselves  in  order  that  the  fourth  may  have  the  bulk, 
that  is  to  say,  the  amount  of  nutritive  material  and  cytoplasm  neces- 
sary for  efficient  development. 

The  Fertilization  of  the  Ovum. — It  is  perfectly  clear  that  the 
reduction  of  the  chromosomes  in  the  germ  cells  cannot  very  long  be 
repeated  in  successive  generations  unless  a  restoration  of  the  original 
number  takes  place  occasionally,  and,  as  a  matter  of  fact,  such  a 
restoration  occurs  at  the  very  beginning  of  the  development  of  each 
individual,  being  brought  about  by  the  union  of  a  spermatozoon 
with  an  ovum.  This  union  constitutes  what  is  known  as  the 
fertilization  of  the  ovum. 

The  fertilization  of  the  human  ovum  has  not  yet  been  observed, 
but  the  phenomenon  has  been  repeatedly  studied  in  lower  forms, 
and  a  thorough  study  of  the  process  has  been  made  on  the  mouse  by 
Sobotta,  whose  observations  are  taken  as  a  basis  for  the  following 
account. 

The  maturation  of  the  ovum  is  quite  independent  of  fertilization, 
but  in  many  forms  the  penetration  of  the  spermatozoon  into  the 
ovum  takes  place  before  the  maturation  phenomena  are  completed. 
This  is  the  case  with  the  mouse.  A  spermatozoon  makes  its  way 
through  the  zona  pellucida  and  becomes  embedded  in  the  cytoplasm 
of  the  ovum  and  its  tail  is  quickly  absorbed  by  the  cytoplasm  while 
its  nucleus  and  probably  the  middle-piece  persist  as  distinct  struc- 
tures. As  soon  as  the  maturation  divisions  are  completed  the  nucleus 
of  the  ovum,  now  termed  the  female  pronucleus  (Fig.  16,  ek),  migrates 
toward  the  center  of  the  ovum,  and  is  now  destitute  of  an  archo- 
plasm  sphere  and  centrosome,  these  structures  having  disappeared 
after  the  completion  of  the  maturation  divisions.  The  spermatozoon 
nucleus,  which,  after  it  has  penetrated  the  ovum,  is  termed  the  male 
pronucleus  (spk),  may  lie  at  first  at  almost  any  point  in  the  peripheral 
part  of  the  cytoplasm,  and  it  now  begins  to  approach  the  female 
pronucleus,  preceded  by  the  middle-piece,  which  becomes  an  archo- 
plasm  sphere  with  its  contained  centrosome  and  is  surrounded  by 


32  THE    FERTILIZATION    OF    THE    OVUM 

astral  rays.  The  two  pronuclei  finally  come  into  contact  near  the 
center  of  the  ovum,  forming  what  is  termed  the  segmentation 
nucleus  (Fig.  16),  and  the  archoplasm  sphere  and  centrosome  which 
have  been  introduced  with  the  spermatozoon  undergo  division  and 
the  two  archoplasm  spheres  so  formed  migrate  to  opposite  poles  of 
the  segmentation  nucleus,  an  amphiaster  forms  and  the  compound 
nucleus  passes  through  the  various  prophases  of  mitosis.  Since, 
in  the  mouse,  the  male  and  female  pronuclei  have  each  contributed 
twelve  chromosomes,  the  equatorial  plate  of  the  mitosis  is  composed 
of  twenty-four  chromosomes,  the  number  characteristic  for  the 
species  being  thus  restored. 

In  describing  the  spermatogenesis  it  was  shown  (p.  16)  that 
two  classes  of  spermatozoa  were  formed,  those  of  one  class  con- 
taining the  equivalent  of  twelve  chromosomes,  while  those  of  the 
other  class  contained  only  ten.  A  similar  separation  of  the  ovum 
into  two  classes  probably  does  not  occur,  the  accessory  chromosomes 
in  the  oocytes  dividing  just  as  do  the  ordinary  ones,  so  that  each 
ovum  possesses  twelve  chromosomes.  When,  therefore,  the  union 
of  the  male  and  female  pronuclei  takes  place  in  fertilization,  those 
ova  that  are  fertilized  by  a  spermatozoon  with  twelve  chromosomes 
will  possess  twenty-four  of  these  bodies,  while  in  those  in  which  the 
fertilization  is  accomplished  by  a  spermatozoon  with  ten  chromo- 
somes, only  twenty-two  will  occur.  The  number  of  chromosomes 
in  the  fertilized  ovum  determines  the  number  in  the  somatic  cells 
of  the  embryo  that  develops  from  it  and  hence  there  will  be  two 
classes  of  embryos,  one  in  which  the  somatic  cells  possess  twenty- 
four  chromosomes  and  another  in  which  there  are  twenty-two. 

That  this  condition  occurs  in  the  human  species  is  at  present 
merely  a  conjecture  based  partly  on  what  occurs  during  spermato- 
genesis and  partly  on  what  has  been  shown  to  occur  in  a  number 
of  invertebrates  (insects).  In  these,  two  classes  of  spermatozoa 
have  been  found  to  occur  as  in  man,  and  two  classes  of  individuals, 
differing  in  the  number  of  chromosomes  in  their  somatic  cells, 
develop  from  the  fertilized  ova;  and  it  has  been  further  found  that 
in  these  forms  those  with  the  greater  number  of  chromosomes 


THE    FERTILIZATION    OF    THE    OVUM 


33 


ek 


Fig.  16. — Six  Stages  in  the  Process  of  Fertilization  of  the  Ovum  of  a  Mouse. 
After  the  first  stage  figured  it  is  impossible  to  determine  which  of  the  two  nuclei 
represents  the  male  or  female  pronucleus,     ek,  Female  pronucleus;  rkl  and  rk2,  polar 
globules;  spk,  male  pronucleus. — (Sobotia.) 

3 


34  THE    FERTILIZATION    OF    THE    OVUM 

become  females  and  those  with  the  smaller  number  males.  If,  as 
seems  probable,  this  condition  also  obtains  in  the  human  species, 
it  is  evident  that  the  sex  of  the  future  individual  is  determined  at 
the  fertilization  of  the  ovum  and  is  correlated  with  the  number  of 
chromosomes  present  in  the  ovum  at  that  stage. 

It  seems  to  be  a  rule  that  but  one  spermatozoon  penetrates  the 
ovum.  Many,  of  course,  come  into  contact  with  it  and  endeavor  to 
penetrate  it,  but  so  soon  as  one  has  been  successful  in  its  endeavor 
no  further  penetration  of  others  occurs.  The  reasons  for  this  are 
in  most  cases  obscure;  experiments  on  the  ova  of  invertebrates  have 
shown  that  the  subjection  of  the  ova  to  abnormal  conditions  which 
impair  their  vitality  favors  the  penetration  of  more  than  a  single 
spermatozoon  {polsypermy),  and,  indeed,  it  appears  that  in  some 
forms,  such  as  the  common  newt  {Diemyctylus) ,  polyspermy  is  the 
rule,  only  one  of  the  spermatozoa,  however,  which  have  penetrated 
uniting  with  the  female  pronucleus,  the  rest  being  absorbed  by  the 
cytoplasm  of  the  ovum. 

Fertilization  marks  the  beginning  of  development,  and  it  is 
therefore  important  that  something  should  be  known  as  to  where 
and  when  it  occurs.  It  seems  probable  that  in  the  human  species  the 
spermatozoa  usually  come  into  contact  with  the  ovum  and  fertilize 
it  in  the  upper  part  of  the  Fallopian  tubes,  and  the  occurrence  of 
extra-uterine  pregnancy  (see  p.  22)  seems  to  indicate  that  occasion- 
ally the  ovum  may  be  fertilized  even  before  it  has  been  received  into 
the  tube. 

It  is  evident,  then,  that  when  fertilization  is  accomplished  the 
spermatozoon  must  have  traveled  a  distance  of  about  twenty-four 
centimeters,  the  length  of  the  upper  part  of  the  vagina  being  taken 
to  be  about  5  cm.,  that  of  the  uterus  as  7  cm.,  and  that  of  the  tube 
as  12  cm.  A  considerable  interval  of  time  is  required  for  the  com- 
pletion of  this  journey,  even  though  the  movement  of  the  spermat- 
ozoon be  tolerably  rapid.  The  observations  of  Henle  and  Hensen 
indicate  that  a  spermatozoon  may  progress  in  a  straight  line  at  about 
the  rate  of  from  1.2  to  2.7  mm.  per  minule,  while  Lott  finds  the  rate 
to  be  as  high  as  3.6  mm.     Assuming  the  rate  of  progress  to  be  about 


THE    FERTILIZATION    OF    THE    OVUM  35 

2.5  mm.  per  minute,  the  time  required  by  the  spermatozoon  to 
travel  from  the  upper  part  of  the  vagina  to  the  upper  part  of  a 
Fallopian  tube  will  be  about  one  and  a  half  hours  (Strassmann). 
This,  however,  assumes  that  there  are  no  obstacles  in  the  way  of  the 
rapid  progress  of  the  spermatozoon,  which  is  not  the  case,  since,  in 
the  first  place,  the  irregularities  and  folds  of  the  lining  membrane 
of  the  tube  render  the  path  of  the  spermatozoon  a  labyrinthine  one, 
and,  secondly,  the  action  of  the  cilia  of  the  epithelium  of  the  tube 
and  uterus  being  from  the  ostium  of  the  tube  toward  the  os  uteri,  it 
will  greatly  retard  the  progress;  furthermore,  it  is  presumable  that 
the  rapidity  of  movement  of  the  spermatozoon  diminishes  after  a 
certain  interval  of  time.  It  seems  probable,  therefore,  that  fertili- 
zation does  not  occur  for  some  hours  after  coition,  even  providing 
an  ovum  is  in  the  tube  awaiting  the  approach  of  the  spermatozoon. 

But  this  condition  is  not  necessarily  present,  and  consequently 
the  question  of  the  duration  of  the  vitality  of  the  sperm  cell  becomes 
of  importance.  Ahlfeld  has  found  that,  when  kept  at  a  proper 
temperature,  a  spermatozoon  will  retain  its  vitality  outside  the  body 
for  eight  days,  and  Diihrssen  reports  a  case  in  which  living  spermat- 
ozoa were  found  in  a  Fallopian  tube  removed  from  a  patient  who 
had  last  been  in  coitu  about  three  and  a  half  weeks  previously. 
As  regards  the  duration  of  the  vitality  of  the  ovum  less  accurate  data 
are  available.  Hyrtl  found  an  apparently  normal  ovum  in  the 
uterine  portion  of  the  left  tube  of  a  female  who  died  three  days  after 
the  occurrence  of  her  second  menstruation,  and  Issmer  estimates 
the  duration  of  the  capacity  for  fertilization  of  an  ovum  to  be  about 
sixteen  days. 

It  is  evident,  then,  that  even  when  the  exact  date  of  the  coitus 
which  led  to  the  fertilization  is  known,  the  actual  moment  of  the 
latter  process  can  only  be  approximated,  and  in  the  immense  ma- 
jority of  cases  it  is  necessary  to  rely  upon  the  date  of  the  last  men- 
struation for  an  estimation  of  the  probable  date  of  parturition. 
And  by  this  method  the  possibilities  for  error  are  much  greater, 
since,  as  been  pointed  out,  ovulation  is  not  necessarily  associated 
with  menstruation.     The  duration  of  pregnancy  is  normally  ten 


36  LITERATURE 

lunar  or  about  nine  calendar  months  and  it  is  customary  to  estimate 
the  probable  date  of  parturition  as  nine  months  and  seven  days 
from  the  last  menstruation.  From  what  has  been  said,  it  is  clear 
that  any  such  estimation  can  be  depended  upon  only  as  an  approxi- 
mation, the  possible  variation  from  it  being  considerable. 

Superf  etation. — The  occasional  occurrence  of  twin  fetuses  in  different 
stages  of  development  has  suggested  the  possibility  of  the  fertilization  of 
a  second  ovum  as  the  result  of  a  coition  at  an  appreciable  interval  of  time 
after  the  first  ovum  has  started  upon  its  development.  There  seems  to 
be  good  reason  for  believing  that  many  of  the  cases  of  supposed  super- 
fetation,  as  this  phenomenon  is  termed,  are  instances  of  the  simultaneous 
fertilization  of  two  ova,  one  of  which,  for  some  cause  concerned  with 
the  supply  of  nutrition,  has  later  failed  to  develop  as  rapidly  as  the  other. 
At  the  same  time,  however,  even  although  the  phenomenon  may  be  of 
rare  occurrence,  it  is  by  no  means  impossible,  for  occasionally  a  second 
Graafian  follicle,  either  in  the  same  or  the  other  ovary,  may  be  so  near 
maturity  that  its  ovum  is  extruded  soon  after  the  first  one,  and  if  the 
development  of  the  latter  and  the  incidental  changes  in  the  uterine  mucous 
membrane  have  not  proceeded  so  far  as  to  prevent  the  access  of  the 
spermatozoon  to  the  ovum,  its  fertilization  and  development  may  ensue. 
The  changes,  however,  which  prevent  the  passage  of  the  spermatozoon 
are  completed  early  in  development  and  the  difference  between  the 
normally  developed  embryo  and  that  due  to  superfetation  will  be  com- 
paratively small,  and  will  become  less  and  less  evident  as  development 
proceeds,  provided  that  the  supply  of  nutrition  to  both  embryos  is  equal. 

LITERATURE. 

E.   Ballowitz:  "  Untersuchungen  iiber   die   Struktur   der   Spermatozoen,"    No.   4. 

Zeitschr.  fiir  wissensch.  Zool.,  lii,  189 1. 
K.  VON  Bardeleben:  "Beitrage  zur  Histologic  des  Hodens  und  zur  Spermatogenese 

beim  Menschen,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,  1897. 
Th.  Boveri:  "Befruchtung,"  Ergebnisse  der  Anat.  und  Entwicklungsgesch.,  I,  1892. 
J.  G.  Clark:  "Ursprung,  Wachsthum  und  Ende  des  Corpus  luteum  nach  Beobach- 

tungen  am  Ovarium  des  Schweines  und  des  Menschen,"  Archiv  filr  Anat.  und 

Physiol.,  Anat.  Abth.,  1898. 
L.  Fraenkel:  "Neue  Experimente  zur  Function  des  Corpus  luteum,"   Arch,  fiir 

Gynaek.,  xci,  1910. 
L.  Gerlach:  "Ueber  die  Bildung  der  Richtungskorper  bei  Mus  museums,"  Wies- 
baden, 1906. 
S.  Gutherz:  "Ueber  ein  bemerkenswertes   Strukturelement  (Heterochromosome)  in 

der  Spermiogenese  des  Menschen,"  Arch.f.  Mikr.  Anat.,  lxxix,  1912.     • 
M.  F,  Guyer:  "Accessory  Chromosomes  in  Man,"  Biol.  Bull.,  xix,  1910. 
W.  Heape:  "The  Sexual  Season  of  Mammals  and  the  Relation  of  the  Procestrum  to 


LITERATURE  37 

Menstruation,"  Quart.  Journ.  Micros.  Sci.,  N.  S.,  xliv,  1901  (contains  very  full 
bibliography) . 
O.   Hertwig:  "Vergleich  der  Ei-   und   Samenbildung  bei   Nematoden,"  Archiv  filr 
mikrosk.  Anat.,  xxxvr,  1890. 

F.  Hitschmann  and  L.  Adler:  "Der  Bau  der  Uterusschleimhaut  des  geschlechts- 

reifen  Weibes,  mit  besonderer  Beriicksichtigung  der  Menstruation,"  Monatsschr. 

filr  Geburtsk.   und  Gynaek.,  xxxn,  1908. 
J.  Jankowski:  "Beitrag  zur  Entstehung  des  Corpus  luteum  der  Saugetiere,"  Arch.  f. 

mikr.  Anat.,  lxiv,  1904. 
W.  B.  Klrkham:  "The  Maturation  of  the  Mouse  Egg,"  Biol.  Bulletin,  xii,  1907. 
H.  Lams  and  J.  Doorme:  "Nouvelles  recherches  sur  la  maturation  et  la  fecondation 

de  1'oeuf  de  mammiferes,"  Arch,  de  Biol.,  xxiii,  1907. 
M.  von  Lenhossek:  "  Untersuchungen  iiber  Spermatogenese,"  Archiv  fiir  mikrosk. 

Anat.,  LI,  1898. 

G.  Leopold  and  A.  Rovano:  "Neuer  Beitrag  zur  Lehre  von  der  Menstruation  und 

Ovulation,"  Arch,  fur  Gynaek.,  Lxxxm,  1907. 
W.  H.  Longley:  "The  Maturation  of  the  Egg  and  Ovulation  in  the  Domestic  Cat," 

Amer.  Journ.  Anat.,  xn,  191 1. 
F.  H.  A.  Marshall:  "The  (Estrus  Cycle  and  the  Formation  of  the  Corpus  luteum  in 

the  Sheep,"  Philos.  Trans.,  Ser.  B,  cxcvi,  1904. 
F.  H.  A.  Marshall:  "The  Development  of  the  Corpus  luteum:  a  Review,"  Quart. 

Journ.  Micros.  Sci.,  N.  S.,  xlix,  1906. 

F.  Meves:  "Ueber  Struktur  und  Histogenese  der  Samenfaden  des  Meerschweinchens," 

Archiv  fiir  mikrosk.  Anat.,  liv,  1899. 
T.  H.  Montgomery:  "Differentiation  of  the  human  Cells  of  Sertoli,"  Biolog.  Bull., 

xxi,  1911. 
W.  Nagel:  "Das  menschliche  Ei,"  Archiv  fiir  mikrosk.  Anat.,  xxxi,  1888. 

G.  Niessing:  "  Die  Betheiligung  der  Centralkorper  und  Sphare  am  Aufbau  des  Samen- 

fadens  bei  Saugethieren,"  Archiv  fiir  mikrosk.  Anat.,  XLvni,  1896. 
G.  Retzixjs:  "Die  Spermien  des  Menschen,"  Biolog.  Untersuch.,  xrv,  1909. 
W.  Rubaschkin:  "Ueber  die  Reifungs- und  Befruchtungs-processe  des  Meerschwein- 

cheneies,"  Anat.  Hefte,  xxix,  1905. 
J.  Sobotta:  "Die  Befruchtung  und  Furchung  des  Eies  der  Maus,"  Archiv  fiir  mikrosk. 

Anat.,  xxy,  1895. 
J.  Sobotta:  "  Ueber  die  Bildung  des  Corpus  luteum  bei  der  Maus,"  Archiv  fiir  mikrosk. 

Anat.,  XL vn,  1897. 
J.  Sobotta:  "Ueber  die  Bildung  des  Corpus  luteum  beim  Meerschweinchen,'M«a<. 

Hefte,  xxxii,  1906. 
J.  Sobotta  and  G.  Burckhard:  "Reifung  und  Befruchtung  der  Eier  des  weissen 

Ratte,"  Anat.  Hefte,  xlii,  1910. 
P.  Strassmann:  "Beitrage  zur  Lehre  von  der  Ovulation,  Menstruation  und  Concep- 
tion," Archiv  fiir  Gynaekol.,  lii,  1896. 
F.  Villemin:  "Le  corps  jaune  considere  comme  glande  a  secretion  interne,"  Paris, 

1908. 
W.  Waldeyer:  "Eierstock  und  Ei,"  Leipzig,  1870. 


CHAPTER  II. 

THE  SEGMENTATION  OF  THE  OVUM  AND  THE  FORMATION 
OF  THE  GERM  LAYERS. 

Segmentation. — The  union  of  the  male  and  female  pronuclei 
has  already  been  described  as  being  accompanied  by  the  formation 
of  a  mitotic  spindle  which  produces  a  division  of  the  ovum  into  two 
cells.  This  first  division  is  succeeded  at  more  or  less  regular 
intervals  by  others,  until  a  mass  of  cells  is  produced  in  which  a 
differentiation  eventually  appears.  These  divisions  of  the  ovum 
constitute  what  is  termed  its  segmentation. 

The  mammalian  ovum  has  behind  it  a  long  line  of  evolution, 
and  even  at  early  stages  in  its  development  it  exhibits  peculiarities 
which  can  only  be  reasonably  explained  as  an  inheritance  of  past 
conditions.  One  of  the  most  potent  factors  in  modifying  the 
character  of  the  segmentation  of  the  ovum  is  the  amount  of  food 
yolk  which  it  contains,  and  it  seems  to  be  certain  that  the  immediate 
ancestors  of  the  mammalia  were  forms  whose  ova  contained  a  con- 
siderable amount  of  yolk,  many  of  the  peculiarities  resulting  from 
its  presence  being  still  clearly  indicated  in  the  early  development  of 
the  almost  yolkless  mammalian  ovum.  To  give  some  idea  of  the 
peculiarities  which  result  from  the  presence  of  considerable  amounts 
of  yolk  it  will  be  well  to  compare  the  processes  of  segmentation  and 
differentiation  seen  in  ova  with  different  amounts  of  it. 

A  little  below  the  scale  of  the  vertebrates  proper  is  a  form, 
Amphioxus,  which  possesses  an  almost  yolkless  ovum,  presenting  a 
simple  process  of  development.  The  fertilized  ovum  of  Amphioxus 
in  its  first  division  separates  into  two  similar  and  equal  cells,  and 
these  are  made  four  (Fig.  17,  A)  by  a  second  plane  of  division 
which  cuts  the  previous  one  at  right  angles.     A  third  plane  at 

38 


THE    SEGMENTATION    OF    THE    OVUM 


39 


right  angles  to  both  the  preceding  ones  brings  about  an  eight-celled 
stage  (Fig.  17,  B),  and  further  divisions  result  in  the  formation 
of  a  large  number  of  cells  which  arrange  themselves  in  the  form 
of  a  hollow  sphere  which  is  known  as  a  blastula  (Fig.  17,  E). 

The  minute  amount  of  yolk  which  is  present  in  the  ovum  of 
Amphioxus  collects  at  an  early  stage  of  the  segmentation  at  one  pole 
of  the  ovum,  the  cells  containing  it  being  somewhat  larger  than  those 
of  the  other  pole  (Fig.  17,  B),  and  in  the  blastula  the  cells  of  one  pole 
are  larger  and  more  richly  laden  with  yolk  than  those  of  the  other 
pole  (Fig.  17,  F).  If,  now,  the  segmenting  ovum  of  an  Amphibian 
be  examined,  it  will  be  found  that  a  very  much  greater  amount  of 


Fig.  17. — Stages  in  the  Segmentation  of  Amphioxus. 

A,  Four-celled  stage;  B,  eight-celled  stage;  C,  sixteen-celled  stage;  D,  early  blastula;  E> 

blastula;  F,  section  of  blastula.- — (Hatschek.) 


yolk  is  present  and,  as  in  Amphioxus,  it  is  located  especially  at  one 
pole  of  the  ovum.  The  first  three  planes  of  segmentation  have  the 
same  relative  positions  as  in  Amphioxus  (Fig.  17),  but  one  of  the 
tiers  of  cells  of  the  eight-celled  stage  is  very  much  smaller  than  the 
other  (Fig.  18,  B).  In  the  subsequent  stages  of  segmentation  the 
small  cells  of  the  upper  pole  divide  more  rapidly  than  the  larger  ones 
of  the  lower  pole,  the  activity  of  the  latter  seeming  to  be  retarded  by 
the  accumulation  of  the  yolk,  and  the  resulting  blastula  (Fig.  18, 


40  THE    SEGMENTATION    OF    THE    OVUM 

D)  shows  a  very  decided  difference  in  the  size  of  the  cells  of  the  two 
poles. 

In  the  ova  of  reptiles  and  birds  the  amount  of  yolk  stored  up  in 
the  ovum  is  very  much  greater  even  than  in  the  amphibia,  and  it  is 
aggregated  at  one  pole  of  the  ovum,  of  which  it  forms  the  principal 
mass,  the  yolkless  protoplasm  appearing  as  a  small  disk  upon  the 


C  D 

Fig.  18. — Stages  in  the  Segmentation  or  Amblystoma. — (Eycleshymer.) 

surface  of  a  relatively  huge  mass  of  yolk.  The  inertia  of  this  mass  of 
nutritive  material  is  so  great  that  the  segmentation  is  confined  to  the 
small  yolkless  disk  of  protoplasm  and  affects  consequently  only  a 
portion  of  the  entire  ovum.  To  distinguish  this  form  of  segmenta- 
tion from  that  which  affects  the  entire  ovum  it  is  termed  meroblastic 
segmentation,  the  other  form  being  known  as  holoblastic. 

In  the  ovum  of  a  turtle  or  a  bird  the  first  plane  of  segmentation 
crosses  the  protoplasmic  disk,  dividing  it  into  two  practically  equal 


THE    SEGMENTATION    OF    THE    OVUM 


41 


halves,  and  the  second  plane  forms  at  approximately  right  angles 
to  the  first  one,  dividing  the  disk  into  four  quadrants  (Fig.  19,  A). 
The  third  division,  like  the  two  which  precede  it,  is  radial  in  position, 
while  the  fourth  is  circular  and  cuts  off  the  inner  ends  of  the  six 
cells  previously  formed  (Fig.  19,  C).  The  disk  now  consists  of 
six  central  smaller  cells  surrounded  by  six  larger  peripheral  ones. 


Fig.   19. — Four    Stages  in  the    Segmentation 

Chick. — (Coste.) 


of    the  Blastoderm  of  the 


Beyond  this  period  no  regularity  can  be  discerned  in  the  appearance 
of  the  segmentation  planes;  but  radial  and  circular  divisions  con- 
tinuing to  form,  the  disk  becomes  divided  into  a  large  number  of 
cells,  those  at  the  center  being  much  smaller  than  those  at  the  per- 
iphery. In  the  meantime,  however,  the  smaller  central  cells  have 
begun  to  divide  in  planes  parallel  to  the  surface  of  the  disk,  which, 
from  being  a  simple  plate  of  cells,  thus  becomes  a  discoidal  cell- 
mass. 


42  THE    SEGMENTATION    OF    THE    OVUM 

During  the  segmentation  of  the  disk  it  has  increased  materially 
in  size,  extending  further  and  further  over  the  surface  of  the  yolk, 
into  the  substance  of  which  some  of  the  lower  cells  of  the  discoidal 
cell-mass  have  penetrated.  A  comparison  of  the  diagram  (Fig. 
20)  of  the  ovum  of  a  reptile  at  about  this  stage  of  development  with 
the  figure  of  the  amphibian  blastula  (Fig.  18,  D)  will  indicate  the 
similarity  between  the  two,  the  large  yolk-mass  ( Y)  of  the  reptile  with 
the  scattered  cells  which  it  contains  corresponding  to  the  lower  pole 


Fig.  20.— Diagram  Illustrating  a  Section  of  the  Ovum  of  a  Reptile  at  a  Stage 
Corresponding  to  the  Blastula  of  an  Amphibian. 
bl,  Blastoderm;  Y,  yolk-mass, 

cells  of  the  amphibian  blastula,  the  central  cavity  of  which  is  prac- 
tically suppressed  in  the  reptile.  Beyond  this  stage,  however,  the 
similarity  becomes  more  obscured.  The  peripheral  cells  of  the  disk 
continue  to  extend  over  the  surface  of  the  yolk  and  finally  complete- 
ly enclose  it,  forming  an  enveloping  layer  which  is  completed  at  the 
upper  pole  of  the  egg  by  the  discoidal  cell-mass,  or,  as  it  is  usually 
termed,  the  blastoderm. 

Turning  now  to  the  mammalia,*  it  will  be  found  that  the  ovum 
in  the  great  majority  is  almost  or  quite  as  destitute  of  food  yolk  as  is 

*  The  segmentation  of  the  human  ovum  has  not  yet  been  observed;  what  follows 
is  based  on  what  occurs  in  the  ovum  of  the  rabbit,  mole,  and  especially  of  a  bat  (Van 
Beneden). 


THE    SEGMENTATION   OF    THE    OVUM 


43 


the  ovum  of  Amphioxus,  with  the  result  that  the  segmentation  is  of 
the  total  or  holoblastic  type.  It  does  not,  however,  proceed  with 
that  regularity  which  marks  the  segmentation  of  Amphioxus  or  an 
amphibian,  but  while  at  first  it  divides  into  two  slightly  unequal 
cells  (Fig.  21),  thereafter  the  divisions  become  irregular,  three-celled, 


Fig.  21. — Four  Stages  in  the  Segmentation  of  the  Ovum  of  a  Mouse. 
X,  Polar  globule.— {Sobolta.) 

four-celled,  five-celled,  and  six-celled  stages  having  been  observed 
in  various  instances.  Nor  is  the  result  of  the  final  segmentation  a 
hollow  vesicle  or  blastula,  but  a  solid  mass  of  cells,  termed  a  morula, 
is  formed.  This  structure  is  not,  however,  comparable  to  the  blas- 
tula of  the  lower  forms,  but  corresponds  to  a  stage  of  reptilian  devel- 
opment a  little  later  than  that  shown  in  Fig.  20,  since,  as  will  be 
shown  directly,  the  cells  corresponding  to  the  blastoderm  and  the 


44  THE    SEGMENTATION   OF    THE    OVUM 

enveloping  layer  are  already  present.  There  is,  then,  no  blastula 
stage  in  the  mammalian  development. 

Differentiation  now  begins  by  the  peripheral  cells  of  the  morula 
becoming  less  spherical  in  shape  and  later  forming  a  layer  of  flat- 
tened cells,  the  enveloping  layer,  surrounding  the  more  spherical 
central  cells  (Fig.  22,  A).  In  the  latter  vacuoles  now  make  their 
appearance,  especially  in  those  cells  which  are  nearest  what  may  be 
regarded  as  the  lower  pole  of  the  ovum  (Fig.  22,  C),  and  these 
vacuoles,  gradually  increasing  in  size,  eventually  become  confluent, 
the  condition  represented  in  Fig.  22,  D,  being  produced.  At  this 
stage  the  ovum  consists  of  an  enveloping  layer,  enclosing  a  cavity 
which  is  equivalent  to  the  yolk-mass  of  the  reptilian  ovum,  the 
vacuolization  of  the  inner  cells  of  the  morula  representing  a  belated 
formation  of  yolk.  On  the  inner  surface  of  the  enveloping  layer, 
at  what  may  be  termed  the  upper  pole  of  the  ovum,  is  a  mass  of  cells 
projecting  into  the  yolk- cavity  and  forming  what  is  known  as  the 
inner  cell-mass,  a  structure  comparable  to  the  blastoderm  of  the 
reptile.  In  one  respect,  however,  a  difference  obtains,  the  inner 
cell-mass  being  completely  enclosed  within  the  enveloping  cells, 
which  is  not  the  case  with  the  blastoderm  of  the  reptile.  That 
portion  of  the  enveloping  layer  which  covers  the  cell-mass  has  been 
termed  Rauber^s  covering  layer,  and  probably  owes  its  existence  to  the 
precocity  of  the  formation  of  the  enveloping  layer. 

It  is  clear,  then,  that  an  explanation  of  the  early  stages  of 
development  of  the  mammalian  ovum  is  to  be  obtained  by  a  com- 
parison, not  with  a  yolkless  ovum  such  as  that  of  Amphioxus,  but 
with  an  ovum  richly  laden  with  yolk,  such  as  the  meroblastic 
ovum  of  a  reptile  or  bird.  In  these  forms  the  nutrition  necessary 
for  the  growth  of  the  embryo  and  for  the  complicated  processes 
of  development  is  provided  for  by  the  storing  up  of  a  quantity 
of  yolk  in  the  ovum,  the  embryo  being  thus  independent  of  external 
sources  for  food.  The  same  is  true  also  of  the  lowest  mammalia, 
the  Monotremes,  which  are  egg-laying  forms,  producing  ova 
resembling  greatly  those  of  a  reptile.  When,  however,  in  the 
higher  mammals  the  nutrition  of  the  embryo  became  provided 


THE   SEGMENTATION   OE   THE    OVUM 


45 


$  ^T?> ... 


<l 


•SI 


V-"A 


'1.      - 


v  ■  ■  ■ , . 


D 


Fig.  22. — Later  Stages  in  the  Segmentation  of  the  Ovum  of  a  Bat. 
A,  C,  and  D  are  sections,  B  a  surface  view. — (Van  Beneden.) 


46  TWIN  DEVELOPMENT  AND    DOUBLE    MONSTERS 

for  by  the  attachment  of  the  embryo  to  the  walls  of  the  uterus 
of  the  parent  so  that  it  could  be  nourished  directly  by  the  parent, 
the  storing  up  of  yolk  in  the  ovum  was  unnecessary  and  it  became  a 
holoblastic  ovum,  although  many  peculiarities  dependent  on  the 
original  meroblastic  condition  persisted  in  its  development. 

Twin  Development. — As  a  rule,  in  the  human  species  but  one  embryo 
develops  at  a  time,  but  the  occurrence  of  twins  is  by  no  means  infrequent, 
and  triplets  and  even  quadruplets  occasionally  are  developed.  The 
occurrence  of  twins  may  be  due  to  two  causes,  either  to  the  simultaneous 
ripening  and  fertilization  of  two  ova,  either  from  one  or  from  both 
ovaries,  or  to  the  separation  of  a  single  fertilized  ovum  into  two  independ- 
ent parts  during  the  early  stages  of  development.  That  twins  may  be 
produced  by  this  latter  process  has  been  abundantly  shown  by  experi- 
mentation upon  developing  ova  of  lower  forms,  each  of  the  two  cells  of  an 
Amphioxus  ovum  in  that  stage  of  development,  if  mechanically  separated, 
completing  its  development  and  producing  an  embryo  of  about  half  the 
normal  size. 

Double  Monsters  and  the  Duplication  of  Parts. — The  occasional 
occurrence  of  double  monsters  is  explained  by  an  imperfect  separation 
into  two  parts  of  an  originally  single  embryo,  the  extent  of  the  separation, 
and  probably  also  the  stage  of  development  at  which  it  occurs,  determining 
the  amount  of  fusion  of  the  two  individuals  constituting  the  monster.  All 
gradations  of  separation  occur,  from  almost  complete  separation,  as  seen 
in  such  cases  as  the  Siamese  twins,  to  forms  in  which  the  two  individuals 
are  united  throughout  the  entire  length  of  their  bodies.  The  separation 
may  also  affect  only  a  portion  of  the  embryo,  producing,  for  instance, 
double-faced  or  double-headed  monsters  or  various  forms  of  so-called 
parasitic  monsters;  and,  finally,  it  may  affect  only  a  group  of  cells  destined 
to  form  a  special  organ,  producing  an  excess  of  parts,  such  as  super- 
numerary digits  or  accessory  spleens. 

It  has  been  observed  in  the  case  of  double  monsters  that  one  of  the 
two  fused  individuals  always  has  the  position  of  its  various  organs  reversed, 
it  being,  as  it  were,  the  looking-glass  image  of  its  fellow.  Cases  of  a 
similar  situs  inversus  viscerum,  as  it  is  called,  have  not  infrequently  been 
observed  in  single  individuals,  and  a  plausible  explanation  of  such  cases 
regards  them  as  one  of  a  pair  of  twins  formed  by  the  division  of  a  single 
embryo,  the  other  individual  having  ceased  to  develop  and  either  having 
undergone  degeneration  or,  if  the  separation  was  an  incomplete  one, 
being  included  within  the  body  of  the  apparently  single  individual. 
Another  explanation  of  situs  inversus  has  been  advanced  (Conklin)  on 
the  basis  of  what  has  been  observed  in  certain  invertebrates.  In  some 
species  of  snails  situs  inversus  is  a  normal  condition  and  it  has  been  found 
that  the  inversion  may  be  traced  back  in  the  development  even  to  the 


FORMATION    OF    THE    GERM    LAYERS  47 

earliest  segmentation  stages.  The  conclusion  is  thereby  indicated  that 
its  primary  cause  may  reside  in  an  inversion  of  the  polarity  of  the  ovum, 
evidence  being  forthcoming  in  favor  of  the  view  that  even  in  the  ovum 
of  these  and  other  forms  there  is  probably  a  distinct  polar  differentiation. 
How  far  this  view  may  be  applicable  to  the  mammalian  ovum  is  uncertain, 
but  if  it  be  applicable  it  explains  the  phenomenon  of  inversion  without 
complicating  it  with  the  question  of  twin-formation. 

The  Formation  of  the  Germ  Layers. — During  the  stages 
which  have  been  described  as  belonging  to  the  segmentation  period 
of  development  there  has  been  but  little  differentiation  of  the  cells. 
In  Amphioxus  and  the  amphibians  the  cells  at  one  pole  of  the  blastula 
are  larger  and  more  yolk-laden  than  those  at  the  other  pole,  and  in 
the  mammals  an  inner  cell-mass  can  be  distinguished  from  the 
enveloping  cells,  this  latter  differentiation  having  been  anticipated  in 
the  reptiles  and  being  a  differentiation  of  a  portion  of  the  ovum  from 
which  alone  the  embryo  will  develop  from  a  portion  which  will  give 


A  B 

Fig.  23. — Two  Stages  in  the  Gastrulation  of  Amphioxus. — {Morgan  and  Hazen.) 

rise  to  accessory  structures.  In  later  stages  a  differentiation  of  the 
inner  cell-mass  occurs,  resulting  first  of  all  in  the  formation  of  a  two- 
layered  or  diploblastic  and  later  of  a  three-layered  or  triploblastic 
stage. 

Just  as  the  segmentation  has  been  shown  to  be  profoundly 
modified  by  the  amount  of  yolk  present  in  the  ovum  and  by  its  sec- 
ondary reduction,  so,  too,  the  formation  of  the  three  primitive  layers 


4S 


FORMATION   OF   THE    GERM   LAYERS 


is  much  modified  by  the  same  cause,  and  to  get  a  clear  understanding 
of  the  formation  of  the  triploblastic  condition  of  the  mammal  it  will 
be  necessary  to  describe  briefly  its  development  in  lower  forms. 

In  Amphioxus  the  diploblastic  condition  results  from  the  flattening 
of  the  large-celled  pole  of  the  blastula  (Fig.  23,  A),  and  finally  from 
the  invagination  of  this  portion  of  the  vesicle  within  the  other  portion 
(Fig.  23 ,  B) .  The  original  single-walled  blastula  in  this  way  becomes 
converted  into  a  double-walled  sac  termed  a  gastrula,  the  outer  layer 
of  which  is  known  as  the  ectoderm  or  epiblast  and  the  inner  layer  as 
the  endoderm  or  hypoblast.  The  cavity  bounded  by  the  endoderm  is 
the  primitive  gut  or  archenteron,  and  the  opening  by  which  this 
communicates  with  the  exterior  is  the  blastopore.  This  last  structure 
is  at  first  a  very  wide  opening,  but  as  development  proceeds  it 

becomes  smaller,  and  finally  is  a 
relatively  small  opening  situated  at 
the  posterior  extremity  of  what 
will  be  the  dorsal  surface  of  the 
embryo. 

As  the  oval  embryo  continues 
to  elongate  in  its  later  development 
the  third  layer  or  mesoderm  makes 
its  appearance.  It  arises  as  a 
lateral  fold  imp)  of  the  dorsal  sur- 
face of  the  endoderm  (en)  on  each 
side  of  the  middle  line  as  indicated 
in  the  transverse  section  shown  in 
Fig.  24.  This  fold  eventually  be- 
comes completely  constricted  off 
from  the  endoderm  and  forms  a 
hollow  plate  occupying  the  space  between  the  ectoderm  and  endo- 
derm, the  cavity  which  it  contains  being  the  body-cavity  or  coelom. 

In  the  amphibia,  where  the  amount  of  yolk  is  very  much  greater 
than  in  Amphioxus,  the  gastrulation  becomes  considerably  modified. 
On  the  line  where  the  large-  and  small-celled  portions  of  the  blastula 
become  continuous  a  crescentic  groove  appears  and,  deepening, 


Fig.  24. — Transverse  Section  of 
A  mphioxus  Embryo  with  Five  Meso- 
derms Pouches. 

Ch,  Notochord;  d,  digestive  cavity; 
ec,  ectoderm;  en,  endoderm;  m,  medul- 
lary plate;  mp,  mesodermic  pouch. — 
(Halschek.) 


FORMATION    OF    THE    GERM    LAYERS 


49 


forms  an  invagination  (Fig.  25,  gc),  the  roof  of  which  is  composed 
of  relatively  small  yolk-containing  cells  while  its  floor  is  formed  by 
the  large  cells  of  the  lower  pole  of  the  blastula.  The  cavity  of  the 
blastula  is  not  sufficiently  large  to  allow  of  the  typical  invagination 
of  all  these  large  cells,  so  that  they  become  enclosed  by  the  rapid 
growth  of  the  ectoderm  cells  of  the  upper  pole  of  the  ovum  over 


Fig.  25. — Section  through  a  Gastrula  of  Amblystoma. 

dl,  Dorsal  lip  of  blastopore;  gc,  digestive  cavity;  gr,  area  of  mesoderm  formation;  mes, 

mesoderm. — (Eycleshymer.) 

them.  Before  this  growth  takes  place  the  blastopore  corresponds 
to  the  entire  area  occupied  by  the  large  yolk  cells,  but  later,  as  the 
growth  of  the  smaller  cells  gradually  encloses  the  larger  ones,  it 
becomes  smaller  and  is  finally  represented  by  a  small  opening 
situated  at  what  will  be  the  hind  end  of  the  embryo. 

Soon  after  the  archenteron  has  been  formed  a  solid  plate  of  cells, 
eventually  splitting  into  two  layers,  arises  from  its  roof  on  each  side 
of  the  median  line  and  grows  out  into  the  space  between  the  ecto- 
derm and  endoderm  (Fig.  26,  mkl  and  mk2),. evidently  corresponding 
to  the  hollow  plates  formed  in  the  same  situations  in  Amphioxus. 
4 


50  FORMATION   OF    THE    GERM    LAYERS 

This  is  not,  however,  the  only  source  of  the  mesoderm  in  the  am- 
phibia, for  while  the  blastopore  is  still  quite  large  there  may  be 
found  surrounding  it,  between  the  endoderm  and  ectoderm,  a  ring  of 
mesodermal  tissue  (Fig.  25,  mes).  As  the  blastopore  diminishes  in 
size  and  its  lips  come  together  and  unite,  the  ring  of  mesoderm 
forms  first  an  oval  and  then  a  band  lying  beneath  the  line  of  closure 
of  the  blastopore  and  united  with  both  the  superjacent  ectoderm 
and  the  subjacent  endoderm.     This  line  of  fusion  of  the  three  germ 


Fig.  26. — Section  through  an  Embryo  Amphibian  (Triton)  of  2%  Days,  showing 
the  Formation  of  the  Gastral  Mesoderm. 
ok,  Ectoderm;  ch,  chorda  endoderm;  dk,  digestive  cavity;  ik,  endoderm;  mk1  and 
mk2,  somatic  and  splanchnic  layers  of  the  mesoderm.     D,  dorsal  and  V,  ventral. — 
(Herlwig.) 

layers  is  known  as  the  primitive  streak.  It  is  convenient  to  distin- 
guish the  mesoderm  of  the  primitive  streak  from  that  formed  from 
the  dorsal  wall  of  the  archenteron  by  speaking  of  the  former  as  the 
prostomial  and  the  latter  as  the  gastral  mesoderm,  though  it  must  be 
understood  that  the  two  are  continuous  immediately  in  front  of  the 
definitive  blastopore. 

In  the  reptilia  still  greater  modifications  are  found  in  the  method 
of  formation  of  the  germ  layers.  Before  the  enveloping  cells  have 
completely  surrounded  the  yolk-mass,  a  crescentic  groove,  resembling 
that  occurring  in  amphibia,  appears  near  the  posterior  edge  of  the 


FORMATION    OF    THE    GERM    LAYERS 


51 


blastoderm,  the  cells  of  which,  in  front  of  the  groove,  arrange  them- 
selves in  a  superficial  layer  one  cell  thick,  which  may  be  regarded  as 
the  ectoderm  (Fig.  27,  ec),  and  a  subjacent  mass  of  somewhat 
scattered  cells.  Later  the  lowermost  cells  of  this  subjacent  mass 
arrange  themselves  in  a  continuous  layer,  constituting  what  is  termed 
the  primary  endoderm  (en1),  while  the  remaining  cells,  aggregated 


prm 


... 


Fig.  27. — Longitudinal  Sections  through  Blastoderms  of  the  Gecko,  showing 

■  Gastrulation. 
ec,  Ectoderm;  en,  secondary  endoderm;  en',  primary  endoderm;  prm,  prostomial  meso- 
derm.— (Will.) 

especially  in  the  region  of  the  crescentic  groove,  form  the  prostomial 
mesoderm  (prm).  In  the  region  enclosed  by  the  groove  a  distinct 
delimitation  of  the  various  layers  does  not  occur,  and  this  region 
forms  the  primitive  streak.  The  groove  now  begins  to  deepen, 
forming  an  invagination  of  secondary  endoderm,  the  extent  of  this 
invagination  being,  however,  very  different  in  different  species. 
In  the  gecko  (Will)  it  pushes  forward  between  the  ectoderm  and 
primary  endoderm  almost  to  the  anterior  edge  of  the  blastoderm 
(Fig.  27,  B),  but  later  the  cells  forming  its  floor,  together  with  those 


52 


FORMATION    OF    THE    GERM    LAYERS 


of  the  primary  endoderm  immediately  below,  undergo  a  degenera- 
tion, the  roof  cells  at  the  tip  and  lateral  margins  of  the  invagination 
becoming  continuous  with  the  persisting  portions  of  the  primary 
endoderm  (Figs.  27,0  and  28,  B) .  This  layer,  following  the  envelop- 
ing cells  in  their  growth  over  the  yolk-mass,  gradually  surrounds 
that  structure  so  that  it  comes  to  lie  within  the  archenteron.  In 
some  turtles,  on  the  other  hand,  the  disappearance  of  the  floor  of  the 
invagination  takes  place  at  a  very  early  stage  of  the  infolding,  the 


en 


Fig.  28. — Diagrams  Illustrating  the  Formation  of  the  Gastral  Mesoderm  in 

the  Gecko. 

ce,  Chorda  endoderm;  ec,  ectoderm;  en,  secondary  endoderm;  en1,  primary  endoderm; 

gm,  gastral  mesoderm. — (Will.) 

roof  cells  only  persisting  to  grow  forward  to  form  the  dorsal  wall  of 
the  archenteron.  This  interesting  abbreviation  of  the  process 
occurring  in  the  gecko  indicates  the  mode  of  development  which  is 
found  in  the  mammalia. 

The  existence  of  a  prostomial  mesoderm  in  connection  with  the 
primitive  streak  has  already  been  noted,  and  when  the  invagination 
takes  place  it  is  carried  forward  as  a  narrow  band  of  cells  on  each 
side  of  the  sac  of  secondary  endoderm.  After  the  absorption  of  the 
ventral  wall  of  the  invagination  a  folding  or  turning  in  of  the  margins 


FORMATION    OF    THE    GERM    LAYERS 


53 


of  the  secondary  endoderrn  occurs  (Fig.  28),  whereby  its  lumen 
becomes  reduced  in  size  and  it  passes  off  on  each  side  into  a  double 
plate  of  cells  which  constitute  the  gastral  mesoderm.     Later  these 


Fig.  29. — Sections  of  Ova  of  a  Bat  showing  (A)  the  Formation  of  the  Endo- 
derm  and  (B  and  C)  of  the  Amniotic  Cavity. — {Van  Beneden.) 

plates  separate  from  the  archenteron  as  in  the  lower  forms.     All  the 
prostomial  mesoderm  does  not,  however,  arise  from  the  primitive 


54  FORMATION    OF    THE    GERM   LAYERS 

streak  region,  but  a  considerable  amount  also  has  its  origin  from 
the  ectoderm  covering  the  yolk  outside  the  limits  of  the  blastoderm 
proper,  a  mode  of  origin  which  serves  to  explain  the  phenomena  later 
to  be  described  for  the  mammalia. 

In  comparison  with  the  amphibians  and  Amphioxus,  the  reptilia 
present  a  subordination  of  the  process  of  invagination  in  the  forma- 
tion of  the  endoderm,  a  primary  endoderm  making  its  appearance 
independently  of  an  invagination,  and,  in  association  with  this 
subordination,  there  is  an  early  appearance  of  the  primitive  streak, 
which,  from  analogy  with  what  occurs  in  the  amphibia,  may  be 
assumed  to  represent  a  portion  of  the  blastopore  which  is  closed 
from  the  very  beginning. 

Turning  now  to  the  mammalia,  it  will  be  found  that  these 
peculiarities  become  still  more  emphasized.  The  inner  cell-mass 
of  these  forms  corresponds  to  the  blastoderm  of  the  reptilian  ovum, 
and  the  first  differentiation  which  appears  in  it  concerns  the  cells 
situated  next  the  cavity  of  the  vesicle,  these  cells  differentiating  to 
form  a  distinct  layer  which  gradually  extends  so  as  to  form  a  com- 
plete lining  to  the  inner  surface  of  the  enveloping  cells  (Fig.  29,  A). 
The  layer  so  formed  is  endodermal  and  corresponds  to  the  primary 
endoderm  of  the  reptiles. 

Before  the  extension  of  the  endoderm  is  completed,  however, 
cavities  begin  to  appear  in  the  cells  constituting  the  remainder  of  the 
inner  mass,  especially  in  those  immediately  beneath  Rauber's  cells 
(Fig.  29,  B),  and  these  cavities  in  time  coalesce  to  form  a  single 
large  cavity  bounded  above  by  cells  of  the  enveloping  layer  and 
below  by  a  thick  plate  of  cells,  the  embryonic  disk  (Fig.  29,  C).  The 
cavity  so  formed  is  the  amniotic  cavity,  whose  further  history  will  be 
considered  in  a  subsequent  chapter. 

It  may  be  stated  that  this  cavity  varies  greatly  in  its  development  in 
different  mammals,  being  entirely  absent  in  the  rabbit  at  this  stage  of 
development  and  reaching  an  excessive  development  in  such  forms  as 
the  rat,  mouse,  and  guinea-pig.  The  condition  here  described  is  that 
which  occurs  in  the  bat  and  the  mole,  and  it  seems  probable,  from  what 
occurs  in  the  youngest  human  embryos  hitherto  observed,  that  the  proc- 
esses in  man  are  closely  similar. 


FORMATION    OF    THE    GERM    LAYERS 


55 


While  these  changes  have  been  taking  place  a  splitting  of  the 
enveloping  layer  has  occurred,  so  that  the  wall  of  the  ovum  is  now 
formed  of  three  layers,  an  outer  one  which  may  be  termed  the 
trophoblast,  a  middle  one  which  probably  is  transformed  into  the 
extra-embryonic  mesoderm  of  later  stages,  though  its  significance 
is  at  present  somewhat  obscure,  and  an  inner  one  which  is  the 


Fig.  30. — A,  Side  View  of  Ovum  of  Rabbit  Seven  Days  Old  {Kdlliker);  B, 
Embryonic  Disk  of  a  Mole  (Heape);  C,  Embryonic  Disk  of  a  Dog's  Ovum  of 
about  Fifteen  Days  (Bonnet) . 

ed,  Embryonic  disk;  hn,  Hensen's  node;  mg,  medullary  groove;  ps,  primitive  streak; 

va,  vascular  area. 


primary  endoderm.  In  the  bat,  of  whose  ovum  Fig.  29,  C,  repre- 
sents a  section,  that  portion  of  the  middle  layer  which  forms  the 
roof  of  the  amniotic  cavity  disappears,  only  the  trophoblast  per- 
sisting in  this  region,  but  in  another  form  this  is  not  the  case,  the 
roof  of  the  cavity  being  composed  of  both  the  trophoblast  and  the 
middle  layer. 

A  rabbit's  ovum  in  which  there  is  yet  no  amniotic  cavity  and  no 
splitting  of  the  enveloping  layer  shows,  when  viewed  from  above, 


56  FORMATION    OF    THE    GERM    LAYERS 

a  relatively  small  dark  area  on  the  surface,  which  is  the  embryonic 
disk.  But  if  it  be  looked  at  from  the  side  (Fig.  30,  A),  it  will  be  seen 
that  the  upper  half  of  the  ovum,  that  half  in  which  the  embryonic 
disk  occurs,  is  somewhat  darker  than  the  lower  half,  the  line  of 
separation  of  the  two  shades  corresponding  with  the  edge  of  the 
primary  endoderm  which  has  extended  so  far  in  its  growth  around 
the  inner  surface  of  the  enveloping  layer.  A  little  later  a  dark  area 
appears  at  one  end  of  the  embryonic  disk,  produced  by  a  prolifera- 
tion of  cells  in  this  region  and  having  a  somewhat  crescentic  form. 
As  the  embryonic  disk  increases  in  size  a  longitudinal  band  makes 
its  appearance,  extending  forward  in  the  median  line  nearly  to  the 
center  of  the  disk,  and  represents  the  primitive  streak  (Fig.  30,  B), 
a  slight  groove  along  its  median  line  forming  what  is  termed  the 
primitive  groove.  In  slightly  later  stages  an  especially  dark  spot 
may  be  seen  at  the  front  end  of  the  primitive  streak  and  is  termed 
Hensen's  node  (Fig.  30,  C,  hn),  while  still  later  a  dark  streak  may 
be  observed  extending  forward  from  this  in  the  median  line  and  is 
termed  the  head-process  of  the  primitive  streak. 


Fig.  31. — Posterior  Portion  of  a  Longitudinal  Section  through  the  Embryonic 

Disk  of  a  Mole. 
bl,  Blastopore,  ec,  ectoderm;  en,  endoderm;  prm,  prostomial  mesoderm. — {After  Heape.) 

To  understand  the  meaning  of  these  various  dark  areas  recourse 
must  be  had  to  the  study  of  sections.  A  longitudinal  section  through 
the  embryonic  disk  of  a  mole  ovum  at  the  time  when  the  crescentic 
area  makes  its  appearance  is  shown  in  Fig.  31.  Here  there  is  to  be 
seen  near  the  hinder  edge  of  the  disk  what  is  potentially  an  opening 
(bl) ,  in  front  of  which  the  ectoderm  (ec)  and  primary  endoderm  (en) 
can  be  clearly  distinguished,  while  behind  it  no  such  distinction  of 


FORMATION    OF    THE    GERM    LAYERS  57 

the  two  layers  is  visible.  This  stage  may  be  regarded  as  compar- 
able to  a  stage  immediately  preceding  the  invagination  stage  of 
the  reptilian  ovum,  and  the  region  behind  the  blastopore  will 
correspond  to  the  reptilian  primitive  streak.  The  later  forward 
extension  of  the  primitive  streak  is  due  to  the  mode  of  growth  of  the 
embryonic  disk.  Between  the  stages  represented  in  Figs.  31  and 
30,  B,  the  disk  has  enlarged  considerably  and  the  primitive  streak 
has  shared  in  its  elongation.  Since  the  blastopore  of  the  earlier 
stage  is  situated  immediately  in  front  of  the  anterior  extremity  of 
the  primitive  streak,  the  point  corresponding  to  it  in  the  older  disk 
is  occupied  by  Hensen's  node,  this  structure,  therefore,  representing 
a  proliferation  of  cells  from  the  region  formerly  occupied  by  the 
blastopore. 

■  — . 


§f# 


M 


WM§\  .  : 


Fig.  32. — Transverse  Section  of  the  Embryonic  Area  of  a  Dog's  Ovum  at  about 

the  Stage  of  Development  shown  in  Fig.  29,  C. 

The  section  passes  through  the  head  process  (Chp);  M,  mesoderm. — (Bonnet.) 

As  regards  the  head  process,  it  is  at  first  a  solid  cord  of  cells 
which  grows  forward  in  the  median  line  from  Hensen's  node,  lying 
between  the  ectoderm  and  the  primary  endoderm.  Later  a  lumen 
appears  in  the  center  of  the  cord,  forming  what  has  been  termed  the 
chorda  canal,  and,  in  some  forms,  including  man,  the  canal  opens  to 
the  surface  at  the  center  of  Hensen's  node.  The  cord  then  fuses 
with  the  subjacent  primary  endoderm  and  then  opens  out  along  the 
line  of  fusion,  becoming  thus  transformed  into  a  flat  plate  of  cells 
continuous  at  either  side  with  the  primary  endoderm  (Fig.  32,  Chp). 
The  portion  of  the  chorda  canal  which  traverses  Hensen's  node  now 


58 


FORMATION    OF    THE    GERM   LAYERS 


opens  below  into  what  will  be  the  primitive  digestive  tract  and  is 
termed  the  neurenteric  canal  (Fig.  t>Z,  nc);  it  eventually  closes  com- 
pletely, being  merely  a  transitory  structure.  The  similarity  of  the 
head  process  to  the  invagination  which  in  the  reptilia  forms  the 
secondary  endoderm  seems  clear,  the  only  essential  difference  being 
that  in  the  mammalia  the  head  process  arises  as  a  solid  cord  which 
subsequently  becomes  hollow,  instead  of  as  an  actual  invagination. 
The  difference  accounts  for  the  occurrence  of  Hensen's  node  and 
also  for  the  mode  of  formation  of  the  neurenteric  canal,  and  cannot 
be  considered  as  of  great  moment  since  the  development  of  what  are 
eventually  tubular  structures  (e.  g.,  glands)  as  solid  cords  of  cells 
which  subsequently  hollow  out  is  of  common  occurrence  in  the 
mammalia.  It  should  be  stated  that  in  some  mammals  apparently 
the  most  anterior  portion  of  the  roof  of  the  archenteron  is  formed 
directly  from  the  cells  of  the  primary  endoderm,  which  in  this  region 
are  not  replaced  by  the  head  process,  but  aggregate  to  form  a  compact 
plate  of  cells  with  which  the  anterior  extremity  of  the  head  process 


Fig.  33. — Diagram  of  a  Longitudinal  Section  through  the  Embryonic  Disk  of 

a  Mole. 
am,  Amnion;  ce  chorda  endoderm;  ec,  ectoderm;  nc,  neurenteric  canal;  ps,  primitive 

streak. — (Heape.) 


unites.     Such  a  condition  would  represent  a  further  modification  of 
the  original  condition. 

As  regards  the  formation  of  the  mesoderm  it  is  possible  to  rec- 
ognize both  the  prostomial  and  gastral  mesoderm  in  the  mammalian 
ovum,  though  the  two  parts  are  not  so  clearly  distinguishable  as  in 
lower  forms.  A  mass  of  prostomial  mesoderm  is  formed  from  the 
primitive  streak,  and  when  the  head  process  grows  forward  it  carries 


FORMATION    OF    THE    GERM    LAYERS 


59 


with  it  some  of  this  tissue.  But,  in  addition  to  this,  a  contribution  to 
the  mesoderm  is  also  apparently  furnished  by  the  cells  of  the  head 
process,  in  the  form  of  lateral  plates  situated  on  each  side  of  the 
middle  line.     These  plates  are  at  first  solid  (Fig.  34,  gm),  but  their 


gm% 


Fig.  34. — Transverse  Section  through  the  Embryonic  Disk  of  a  Rabbit. 
ch,  Chorda  endoderm;  ee,  ectoderm;  en,  endoderm;  gm,  gastral  mesoderm. — {After  van 

Beneden.) 


Fig.  35.- — Diagrams  Illustrating  the  Relations  of  the  Chick  Embryo  to  the 
Primitive  Streak  at  Different  Stages  of  Development. — (Peebles.) 

cells  quickly  arrange  themselves  in  two  layers,  between  which  a 
ccelomic  space  later  appears. 

Furthermore,   as  has  already  been  pointed  out,   the  layer  of 


60  SIGNIFICANCE    OF    THE    GERM    LAYERS 

enveloping  cells  splits  into  two  concentric  layers,  the  inner  of  which 
seems  to  be  mesodermal  in  its  nature  and  forms  a  layer  lining  the 
interior  of  the  trophoblast  and  lying  between  this  and  the 
primary  endoderm.  This  layer  is  by  no  means  so  evident  in  the 
lower  forms,  but  is  perhaps  represented  in  the  reptilian  ovum  by  the 
cells  which  underlie  the  ectoderm  in  the  regions  peripheral  to  the 
blastoderm  proper  (see  p.  54). 

It  has  been  experimentally  determined  (Assheton,  Peebles)  that  in 
the  chick,  whose  embryonic  disk  presents  many  features  similar  to  those 
of  the  mammalian  ovum,  the  central  point  of  the  unincubated  disk  corre- 
sponds to  the  anterior  end  of  the  primitive  streak  and  to  the  point  situated 
immediately  behind  the  heart  of  the  later  embryo  and  immediately  in 
front  of  the  first  mesodermic  somite  (see  p.  77),  as  shown  in  Fig.  35.  If 
these  results  be  regarded  as  applicable  to  the  human  embryo,  then  it 
may  be  supposed  that  in  this  the  head  region  is  developed  from  the 
portion  of  the  embryonic  disk  situated  in  front  of  Hensen's  node,  while 
the  entire  trunk  is  a  product  of  the  region  occupied  by  the  node. 

The  Significance  of  the  Germ  Layers. — The  formation  of 
the  three  germ  layers  is  a  process  of  fundamental  importance,  since 
it  is  a  differentiation  of  the  cell  units  of  the  ovum  into  tissues  which 
have  definite  tasks  to  fulfil.  As  has  been  seen,  the  first  stage  in  the 
development  of  the  layers  is  the  formation  of  the  ectoderm  and 
endoderm,  or,  if  the  physiological  nature  of  the  layers  be  considered, 
it  is  the  differentiation  of  a  layer,  the  endoderm,  which  has  princi- 
pally nutritive  functions.  In  certain  of  the  lower  invertebrates,  the 
class  Ccelentera,  the  differentiation  does  not  proceed  beyond  this 
diploblastic  stage,  but  in  all  higher  forms  the  intermediate  layer  is 
also  developed,  and  with  its  appearance  a  further  division  of  the 
functions  of  the  organism  supervenes,  the  ectoderm,  situated  upon 
the  outside  of  the  body,  assuming  the  relational  functions,  the 
endoderm  becoming  still  more  exclusively  nutritive,  while  the  remain- 
ing functions,  supportive,  excretory,  locomotor,  reproductive,  etc., 
are  assumed  by  the  mesoderm. 

The  manifold  adaptations  of  development  obscure  in  certain 
cases  the  fundamental  relations  of  the  three  layers,  certain  portions 
of  the  mesoderm,  for  instance,  failing  to  differentiate  simultaneously 


SIGNIFICANCE    OF    THE    GERM    LAYERS  6 1 

with  the  rest  of  the  layer  and  appearing  therefore  to  be  a  portion  of 
either  the  ectoderm  or  endoderm.  But,  as  a  rule,  the  layers  are 
structural  units  of  a  higher  order  than  the  cells,  and  since  each 
assumes  definite  physiological  functions,  definite  structures  have 
their  origin  from  each. 

Thus  from  the  ectoderm  there  develop: 
i.  The  epidermis  and  its  appendages,  hairs,  nails,  epidermal 
glands,  and  the  enamel  of  the  teeth. 

2.  The  epithelium  lining  the  mouth  and  the  nasal  cavities,  as 
well  as  that  lining  the  lower  part  of  the  rectum. 

3.  The  nervous  system  and  the  nervous  elements  of  the  sense- 
organs,  together  with  the  lens  of  the  eye. 

From  the  endoderm  develop : 

1.  The  epithelium  lining  the  digestive  tract  in  general,  together 
with  that  of  the  various  glands  associated  with  it,  such  as  the  liver 
and  pancreas. 

2.  The  lining  epithelium  of  the  larynx,  trachea,  and  lungs. 

3.  The  epithelium  of  the  bladder  and  urethra  (in  part). 
From  the  mesoderm  there  are  formed: 

1.  The  various  connective  tissues,  including  bone  and  the  teeth 
(except  the  enamel). 

2.  The  muscles,  both  striated  and  non-striated. 

3.  The  circulatory  system,  including  the  blood  itself  and  the 
lymphatic  system. 

4.  The  lining  membrane  of  the  serous  cavities  of  the  body. 

5.  The  kidneys  and  ureters. 

6.  The  internal  organs  of  reproduction. 

From  this  list  it  will  be  seen  that  the  products  of  the  mesoderm 
are  more  varied  than  those  of  either  of  the  other  layers.  Among 
its  products  are  organs  in  which  in  either  the  embryonic  or  adult 
condition  the  cells  are  arranged  in  a  definite  layer,  while  in  other 
structures  its  cells  are  scattered  in  a  matrix  of  non-cellular  material, 
as,  for  example,  in  the  connective  tissue,  bone,  cartilage,  and  the 
blood  and  lymph.  It  has  been  proposed  to  distinguish  these  two 
forms  of  mesoderm  as  mesothelium  and  mesenchyme  respectively, 


62  LITERATURE 

a  distinction  which  is  undoubtedly  convenient,  though  probably  de- 
void of  the  fundamental  importance  which  has  been  attributed  to  it 
by  some  embryologists. 

LITERATURE. 

R.  Assheton:  "The  Reinvestigation  into  the  Early  Stages  of  the  Development  of 

the  Rabbit,"  Quarterly  Journ.  of  Microsc.  Science,  xxxvn,  1894. 
R.  Assheton:  "The  Development  of  the  Pig  During  the  First  Ten  Days,"  Quarterly 

Journ.  of  Microsc.  Science,  xli,  1898. 
R.  Assheton:  "The  Segmentation  of  the  Ovum  of  the- Sheep,  with  Observations  on 

the  Hypothesis  of  a  Hypoblastic  Origin  for  the  Trophoblast,"  Quarterly  Journ. 

of  Microsc.  Science,  xli,  1898. 
E.  van  Beneden:  "Recherches  sur  les  premiers  stades  du  developpement  du  Murin 

(Vespertilio  murinus),"  Anatom.  Anzeiger,  xvi,  1899. 
R.  Bonnet:  "Beitrage  zur  Embryologie  der  Wiederkauer  gewonnen  am  Schafei," 

Archivfiir  Anat.  und  Physiol.,  Anat.  Abth.,  1884  and  1889. 
R.  Bonnet:  "Beitrage  zur  Embryologie  des  Hundes,"  Anat.  Hefte,  ix,  1897. 
G.  Born:  "Erste  Entwickelungsvorgange,"  Ergebnisse  der  Anat.  und  Entwicklungs- 

gesch.,  1,  1892. 

E.  G.  Conklin:  "The  Cause  of  Inverse  Symmetry,"  Anatom.  Anzeiger,  xxm,  1903. 

A.  C.  Eycleshymer:  "The  Early  Development  of  Amblystoma  with  Observations 

on  Some  Other  Vertebrates,"  Journ.  of  Morphol.,  x,  1895. 

B.  Hatschek:  "Studien  uber  Entwicklung  des  Amphioxus,"  Arbeiten  aus  dem  zoolog. 

Ins  tit.  zu  Wien,  rv,  1881. 
W.  Heape:  "The  Development  of  the  Mole  (Talpa  europaea),"  Quarterly  Journ.  of 

Microsc.  Science,  xxm,  1883. 
A.  A.  W.  Hubrecht:  "Studies  on  Mammalian  Embryology  II:  The  Development 

of  the  Germinal  Layers  of  Sorex  vulgaris,"  Quarterly  Journ.  of  Microsc.  Science, 

xxxi,  1890. 

F.  Keibel:  "Studien    zur    Entwicklungsgeschichte    des    Schweines,"     Morpholog. 

Arbeiten,  in,  1893. 
F.  Keibel:  "Die  Gastrulation  und  die  Keimblattbildung  der  Wirbeltiere,"  Ergebnisse 

der  Anat.  und  Entwicklungsgesch.,  x,  1901. 
M.  KunsemVJller:  "Die  Eifurchung  des  Igels  (Erinaceus  europasus  L.),"  Zeitschr. 

fiir  wissensch.  Zool.,  lxxxv,  1906. 
K.  Mitsukuri  and  C.  Ishikawa:  "On  the  Formation  of  the  Germinal  Layers  in 

Chelonia,"  Quarterly  Journ.  of  Microsc.  Science,  xxvn,  1887. 
F.  Peebles:  "The  Location  of  the  Chick  embryo  upon  the  Blastoderm,"  Journ.  of 

Exper.  Zool.,  1,  1904. 
E.  Selenka:  "  Studien  uber  Entwickelungsgeschichte  der  Thiere,"  4tes  Heft,  1886-87; 

5tes  Heft,  1891-92. 
J.  Sobotta:  "DieBefruchtungundFurchungdesEies  der  Maus,"  Archivfiir  mikrosk. 

Anat.,  xlv,  1895. 


LITERATURE  63 

J.  Sobotta:  "  Die  Furchung  des  Wirbelthiereies,"  Ergebnisse  der  Anal,  unci  Entwicke- 

lungsgeschichte,  vi,  1897. 
J.   Sobotta:  "Neuere  Auschauungen  iiber  die  Entstehung  der  Doppel  (miss)  bild- 

ungen,    mit  besonderer  Beriicksichtigung  der  menschlichen  Zwillingsgeburten," 

Wiirzburger  Abhandl.,  I,  1901. 
H.  H.  Wilder:  "Duplicate  Twins  and  Double  Monsters,"  Amer.  Jour,  of  Anal., 

in,  1904. 
L.  Will:  "Beitrage  zur  Entwicklungsgeschichte  der  Reptilien,"  Zoolog.  Jahrbilcher 

Abth.fur  Anal.,  vi,  1893. 


CHAPTER  III. 

THE  MEDULLARY  GROOVE,  NOTOCHORD,  AND  MESO- 
DERMS SOMITES. 

In  the  preceding  chapter  the  development  of  the  mammalian 
ovum  has  been  described  up  to  and  including  the  formation  of  the 
three  germinal  layers.  The  earlier  stages  of  development  there 
described  are  practically  unknown  in  the  human  ovum,  but  for  the 
stages  subsequent  to  the  establishment  of  the  germinal  layers 
human  material  is  available,  and  it  will,  therefore,  now  be  con- 
venient to  consider  the  structure  of  the  younger  human  ova  at 
present  known  and  to  trace  in  them  the  appearance  and  develop- 
ment of  such  structures  as  the  primitive  streak,  the  head  process  and 
the  gastral  mesoderm. 

The  youngest  human  ovum  at  present  known  is  that  described 
by  Bryce  and  Teacher,  but,  unfortunately,  it  presents  certain 
features  that  are  evidently  abnormal,  so  that  it  becomes  doubtful 
how  far  it  may  be  accepted  as  representing  the  typical  condition. 
The  trophoblast,  which  was  very  thick  and  clearly  differentiated 
into  two  layers,  enclosed  a  space  whose  diameter  was  about  0.63 
mm.  and  which  was  largely  occupied  by  a  loose  syncytial  tissue, 
presumably  mesoderm.  Toward  the  center  of  this  was  an  irregular 
cavity  in  which  were  two  vesicles,  quite  separate  from  one  another 
and  probably  together  representing  the  embryo,  the  smaller  one 
being  the  amniotic  cavity  and  the  larger  one  the  yolk-sac  (Fig.  36). 
The  separation  of  these  two  structures  is  apparently  an  abnormality 
and  it  is  possible  that  the  cavity  in  which  they  lie  is,  as  Bryce  and 
Teacher  suggest,  an  artefact  produced  by  contraction  of  the  syncytial 
mesoderm  during  the  preservation  of  the  ovum. 

If  comparison  of  this  ovum  with  those  of  other  mammals  is 
warranted,  it  may  be  likened  to  that  of  the  bat  as  shown  in  Fig.  29, 

64 


THE   MEDULLARY    GROOVE 


65 


C,  with  the  difference  that  the  mesoderm  that  lines  the  trophoblast 
in  that  ovum  has  become  much  more  voluminous  and  forms  the 
syncytial  mass  in  which  the  ovum  is  supposed  to  have  been  imbedded, 
a  condition  that  may  be  "represented  diagrammatically  as  in  Fig. 
38,  A. 

Somewhat  older  are  the  ova  described  by  Peters,  Fetzer,  Jung 
and   Herzog.     The  Peters  ovum  was  taken  from  the  uterus  of  a 


Fig.  36. 


-From  a  Reconstruction  of  the  Bryce-Teacher  Ovum. — 
(Bryce-Teacher .) 


woman  who  had  committed  suicide  one  calendar  month  after  the 
last  menstruation,  and  it  measured  about  1  mm.  in  diameter.  The 
entire  inner  surface  of  the  trophoblast  (Fig.  37,  ce)  was  lined  by  a 
layer  of  mesoderm  {cm),  which,  on  the  surface  furthest  away  from 
the  uterine  cavity,  was  considerably  thicker  than  elsewhere,  forming 
an  area  of  attachment  of  the  embryo  to  the  wall  of  the  ovum.  In 
the  substance  of  this  thickening  was  the  amniotic  cavity  (am), 
whose  roof  was  formed  by  flattened  cells,  which,  at  the  sides,  became 
continuous  with  a  layer  of  columnar  cells  forming  the  floor  of  the 
cavity  and  constituting  the  embryonic  ectoderm  (ec).  Immediately 
5 


66 


THE    MEDULLARY   GROOVE 


below  this  was  a  layer  of  mesoderm  (m)  which  split  at  the  edge  of 
the  embryonic  disk  into  two  layers,  one  of  which  became  continuous 
with  the  mesodermic  thickening  and  so  with  the  layer  of  mesoderm 
lining  the  interior  of  the  trophoblast,  while  the  other  enclosed  a  sac 
lined  by  a  layer  of  endodermal  cells  and  forming  the  yolk-sac  (ys). 
The  total  length  of  the  embryo  was  0.19  mm.,  and  so  far  as  its 
ectoderm  and  mesoderm  are  concerned  it  might  be  described  as  a 


cm- 


<r  \ 


* 


* 


1  *-5SC§*  ^k  « m 


Fig.  37. — Section  of  Embryo  and  Adjacent  Portion  of  an  Ovum  of  i  mm. 

am,  Amniotic  cavity;  ce,  chorionic  ectoderm;  cm,  chorionic  mesoderm;  ec,  embryonic 

ectoderm;  en,  endoderm;  m,  embryonic  mesoderm;  ys,  yolk-sack. — (Peters.) 


flat  disk  resting  on  the  surface  of  the  yolk-sac,  though  it  must  be 
understood  that  the  yolk-sac  also  to  a  certain  extent  forms  part  of 
the  embryo. 

This  embryo  seems  to  be  in  an  early  stage  of  the  primitive  streak 
formation,  before  the  development  of  the  head  process.  On  com- 
paring it  with  the  stage  of  development  represented  in  Fig.  38,  A, 
it  will  be  seen  to  present  some  important  advances.  The  cavity 
(Fig.  38,  B,  C)  into  which  the  yolk-sac  projects  is  unrepresented  in 


THE    MEDULLARY    GROOVE 


67 


Fig.  38,  A.  How  this  cavity  is  formed  can  only  be  conjectured,  but 
it  seems  probable  that  it  arises  by  the  splitting  of  the  layer  of  cells 
which  lines  the  interior  of  the  trophoblast  in  the  earlier  stage  (or 
perhaps  by  the  vacuolization  of  the  central  cells  of  this  layer)  and 
the  subsequent  accumulation  of  fluid  between  the  two  meso- 
dermal layers  so  formed.  However  that  may  be,  it  seems  clear  that 
the  size  of  the  human  ovum  is  due  mainly  to  the  rapid  growth  of 
this  cavity,  which,  as  future  stages  show,  is  the  extra-embryonic 
portion  of  the  body-cavity,  the  splitting  or  vacuolization  of  the 


Fig.  38. — Diagrams  to  show  the  Probable  Relationships  of  the  Parts  in  the 
Embryos  Represented  in  Figs.  29,  C,  and  37. 
Ac,  Amniotic  cavity;  C,  extra-embryonic  body-cavity;  Me,  (in  figure  to  the  left) 
mesoderm,  (in  figure  to  the  right)  somatic  mesoderm;  Me,  splanchnic  mesoderm;  D, 
digestive  tract;  En,  endoderm;  T,  trophoblast.  The  broken  line  in  the  mesoderm  of  the 
figure  to  the  left  indicates  the  line  along  which  the  splitting  of  the  mesoderm  occurs. 


mesoderm  by  which  it  is  probably  formed  being  the  precocious 
appearance  of  the  typical  splitting  of  the  mesoderm  to  form  the 
embryonic  body-cavity  which,  as  will  be  seen  in  a  subsequent  chap- 
ter, takes  place  only  at  a  later  stage  of  development.  From  now  on 
the  trophoblast  and  the  layer  of  mesoderm  lining  it  may  together 
be  spoken  of  as  the  chorion,  the  mesoderm  layer  being  termed  the 
chorionic  mesoderm. 

A  little  older  again  than  the  Peters  and  Herzog  ova  are  those 
described  by  Strahl  and  Beneke  and  by  von  Spee  (Embryo  v.  H.), 
the  chorionic  cavity  of  the  former  two  having  an  average  diameter 


68 


THE   MEDULLARY   GROOVE 


of  about  2.4  mm.,  while  the  corresponding  size  of  the  latter  two  was 
somewhat  less  than  4.0  mm.  Notwithstanding  the  considerable 
increase  in  the  size  of  these  older  ova,  due  to  the  continued  increase 
in  the  size  of   the  extra-embryonic  ccelom,  the  embryos  are  but 


Fig.  39. — The  Embryo  v.  H.  of  von  Spee.    The  Left  Half  of  theT  Chorion  has 

been  Removed  to  show  the  Embryo. 
a,  Amniotic  cavity;  b,  belly-stalk;  ch,  chorion;  d,  yolk-sac;  e,  extra-embryonic  ccelom; 
ky  embryonic  disk;  2,  chorionic  villus. — {von Spee.) 

little  advanced  beyond  the  stage  shown  by  the  Peters  embryo. 
The  thickening  of  the  chorionic  mesoderm  that  encloses  the  amni- 
otic cavity  has  increased  in  size  and  now  forms  a  pedicle,  known  as 
the  belly-stalk  (Fig.  39,  6),  at  the  extremity  of  which  is  the  yolk-sac 


Fig.  40. — Embryo  from  the  Beneke  Ovum,  the  Roof  of  the  Amniotic  Cavity 

having  been  Removed. 
From  a  model,     b,  Belly-stalk;  p.g.,  primitive  groove;  y,  yolk-sac  — {Strahl  and  Beneke.) 

(d).  Furthermore,  the  amniotic  cavity  (a)  now  lies  somewhat  excen- 
trically  in  this  pedicle,  being  near  what  may  be  termed  its  anterior 
surface,  and  the  entire  embryo  projects  like  a  papilla  from  the  inner 
surface  of  the  chorion  into  the  extra-embryonic  ccelom.    Fig.  40  is 


THE    MEDULLARY    GROOVE  69 

from  a  model  of  the  Beneke  embryo,  detached  from  the  chorion  by 
cutting  through  the  belly-stalk,  and  with  the  roof  of  the  amniotic 
cavity  removed.  The  dorsal  surface  of  the  embryo,  thus  exposed, 
is  an  oval  disk,  resting,  as  it  were,  on  the  yolk-sac,  and  quite  smooth 
except  for  a  slight  longitudinal  groove  upon  its  posterior  portion. 
This  is  the  primitive  groove  and  sections  passing  through  it  show  the 
primitive  streak,  consisting  of  a  sheet  of  mesoderm  interposed 
between  the  ectoderm  and  endoderm,  as  in  the  Peters  embryo,  and 
but  poorly  defined  from  the  other  two  layers.  From  its  anterior 
edge  a  median  process  extends  forward  for  a  short  distance  and  is 
the  head  process  (see  p.  56).  In  front  and  to  the  sides  of  this  there 
is  as  yet  no  mesoderm  intervening  between  the  ectoderm  and 
endoderm. 


Fig.  41. — Embryo  from  the  Frassi  Ovum,  the  Roof  of  the  Amniotic  Cavity 

having  been  removed. 
From  a  model,     b,  belly-stalk;  p.g.,  primitive  groove;  mg,  medullary  groove;  n,  neuren- 

teric  canal. — (Frassi.) 

The  embryonic  disk  of  the  Beneke  embryo  measured  0.75  mm. 
in  length.  That  of  an  embryo  described  by  Frassi  (Fig.  41)  was 
1. 1 7  mm.  in  length,  and  in  correspondence  with  its  greater  size,  it 
presents  some  advances  in  structure  that  are  of  interest.  As  in 
the  younger  embryo  one  sees  a  distinct  primitive  groove  on  the 
posterior  portion  of  the  embryonic  disk,  but  the  groove  terminates 
anteriorly  at  a  distinct  pore  (w) ,  which  perforates  the  disk  and  opens 
ventrally  into  the  yolk-sac.  This  is  the  neurenteric  canal  (see  p.  58) 
and  in  front  of  it  a  groove  extends  forward  in  the  median  line  almost 
to  the  anterior  edge  of  the  embryonic  disk  and  is  evidently  the  first 


7° 


THE    MEDULLARY   GROOVE 


indication  of  the  medullary  groove,  whose  walls  are  destined  to  give 
rise  to  the  central  nervous  system.  Sections  passing  through  the 
region  of  the  medullary  groove  show,  lying  beneath  it,  the  head 
process  (Fig.  42,  hp),  already  fused  with  the  endoderm  (compare 
p.  57),  and  on  each  side  of  the  process  is  a  plate  of  mesoderm  (gm), 
representing  the  gastral  mesoderm  of  lower  forms  (see  Figs.  28 
and  34) ,  but  not  as  yet  showing  any  indications  of  splitting  into  the 
two  layers  that  bound  the  embryonic  ccelom  (see  p.  59). 


am 


Fig.  42. — Section  through  the  Frassi  Embryo  just  in  Front  of  the  Neuren- 

teric  Canal. 
am,  Amniotic  cavity;  gm,  gastral  mesoderm;  hp,  head  process;  mp,  medullary  plate;  ys> 

yolk-sac. — (Frassi.) 


This  is  just  beginning  to  appear  in  an  embryo,  also  described  by 
von  Spee  and  known  as  embryo  Gle.  It  measured  1.54  mm.  in 
length  and  is  closely  similar,  in  general  appearance,  to  an  embryo 
described  by  Eternod  and  measuring  1.34  mm.  in  length  (Fig.  43). 
It  differs  from  the  Frassi  embryo  most  markedly  in  that  the  posterior 
portion  of  the  embryonic  disk,  that  is  to  say  the  primitive  streak 
region,  is  bent  ventrally  so. as  to  lie  almost  at  a  right  angle  with  the 
anterior  portion.  As  a  result  the  belly-stalk  arises  from  the  ventral 
surface  of  the  embryo  instead  of  from  its  posterior  extremity,  near 
which  the  opening  of  the  neurenteric  canal  (Fig.  43,  nc)  is  now  situ- 
ated, almost  the  whole  length  of  the  surface  seen  in  dorsal  view 
being  occupied  by  the  medullary  groove  (m),  which,  in  the  embryo 
Gle,  is  bounded  laterally  by  distinct  ridges,  the  medullary  folds. 


THE    MEDULLARY    GROOVE 


71 


Fig.  43. — Embryo  1.34  mm.  Long. 

al   Allantois;  am,  amnion;  bs,  belly-stalk;  h,  heart;  m,  medullary  groove;  tic  neuren 

tenc  canal;  pc,  caudal  protuberance;  ps,  primitive  streak;  ys,  yolk-stalk.— (Eternod.) 


72 


THE    MEDULLARY   FOLDS 


In  the  Kromer  embryo  Klb  (Fig.  44),  measuring  i.8  mm.  in 
length,  a  new  feature  has  made  its  appearance.  The  medullary  folds 
have  become  quite  high,  and  lateral  to  them  there  is  on  each  side 
a  series  of  five  or  six  oblong  elevations,  which  represent  what  are 
termed  mesodermic  somites  and  are  due  to  divisions  of  the  under- 
lying mesoderm. 


Fig.  44. — Model  of  the  Kromer  Embryo  Klb  seen  from  the  Dorsal  Surface,  the 
Roof  of  the  Amniotic  Cavity  having  been  Removed. — (Keibel  and  Elze.) 

Instead  of  proceeding  with  a  description  of  the  external  form  of 
still  older  embryos  it  will  be  convenient  to  consider  the  further 
development  of  certain  structures  whose  appearance  has  already 
been  noted,  namely,  the  head  process,  the  medullary  folds  and  the 
mesodermic  somites,  and  first  of  all  •  the  medullary  folds  may  be 
considered. 

The  Medullary  Folds. — The  two  folds  are  continuous  anteriorly, 
but  behind  they  are  at  first  separate,  the  anterior  portion  of  the  primi- 
tive streak  lying  between  them.  In  forms,  such  as  the  Reptilia, 
which  possess  a  distinct  blastopore,  this  opening  lies  in  the  interval 
between  the  two,  and  consequently  is  in  the  floor  of  the  medullary 
groove,  and  in  the  mammalia,  even  though  no  well-defined  blastopore 
is  formed,  yet  at  the  time  of  the  formation  of  the  medullary  fold  an 
opening  breaks  through  at  the  anterior  end  of  the  primitive  streak 
in  the  region  of  Hensen's  node,  and  places  the  cavity  lying  below 
the  endoderm  in  communication  with  the  space  bounded  by  the 
medullary  folds.     The  canal  so  formed  is  termed  the  neurenteric 


THE   MEDULLARY    FOLDS 


73 


canal  (Figs.  43  and  45,  nc)  and  is  so  called  because  it  unites  what 
will  later  become  the  central  canal  of  the  nervous  system  with  the 
intestine  (enteron).  The  significance  of  this  canal  has  already  been 
discussed  (p.  58) ;  it  is  of  very  brief  persistence,  closing  at  an  early 
stage  of  development  so  as  to  leave  no  trace  of  its  existence. 


Fig.  45. — Diagram  of  a  Longitudinal  Section  through  the  Embryo  Gle,  Meas- 
uring 1.54  mm.  in  Length. 
al,  Allantois;  am,  amnion;  B,  belly-stalk;  ch,  chorion;  h,  heart;  nc,  neurenteric  canal;  V, 
chorionic  villi;  Y,  yolk-sac. — (vonSpee.) 


As  development  proceeds  the  medullary  folds  increase  in  height 
and  at  the  same  time  incline  toward  one  another  (Fig.  44),  so  that 
their  edges  finally  come  into  contact  and  later  fuse,  the  two  ecto- 
dermal layers  forming  the  one  uniting  with  the  corresponding  layers 
of  the  other  (Fig.  46).  By  this  process  the  medullary  groove  be- 
comes converted  into  a  medullary  canal  which  later  becomes  the 


74  THE    NOTOCHORD 

central  canal  of  the  spinal  cord  and  the  ventricles  of  the  brain,  the 
ectodermal  walls  of  the  canal  thickening  to  give  rise  to  the  central 
nervous  system.  The  closure  of  the  groove  does  not,  however,  take 
place  simultaneously  along  its  entire  length,  but  begins  in  what 
corresponds  to  the  neck  region  of  the  adult  and  thence  proceeds  both 


Fig.  46. — Diagrams  showing  the  Manner  of  the  Closure  of  the  Medullary 

Groove. 

anteriorly  and  posteriorly,  the  extension  of  the  fusion  taking  place 
rather  slowly,  however,  especially  anteriorly,  so  that  an  anterior 
opening  into  the  otherwise  closed  canal  can  be  distinguished  for  a 
considerable  period  (Fig.  53). 

The  Noto chord. — While  these  changes  have  been  taking  place  in 
the  ectoderm  of  the  median  line  of  the  embryonic  disk,  modifications 
of  the  subjacent  endoderm  have  also  occurred.  This  endoderm, 
it  will  be  remembered,  was  formed  by  the  head  process  of  the  primi- 
tive streak,  and  was  a  plate  of  cells  continuous  at  the  sides  with  the 
primary  endoderm  and  extending  forward  as  far  as  what  will  eventu- 
ally be  the  anterior  part  of  the  pharynx.  Along  the  line  of  its 
junction  with  the  primary  endoderm  it  gives  rise  to  the  plates  of 
gastral  mesoderm  (Fig.  28),  while  the  remainder  of  it  produces  an 


THE    NOTOCHORD 


75 


important  embryonic  organ  known  as  the  notochord  or  chorda  dorsalis 
and  on  this  account  is  sometimes  termed  the  chorda  endoderm. 

After  the  separation  of  the  plates  of  gastral  mesoderm  the  chorda 
endoderm,  which  is  at  first  a  flat  band,  becomes  somewhat  curved 
(Fig.  47,  A),  so  that  it  is  concave  on  its  under  surface,  and,  the  curva- 
ture increasing,  the  edges  of  the  plate  come  into  contact  and  finally 
fuse  together  (Fig.  47,  B),  the  edges  of  the  primary  endoderm  at  the 
same  time  uniting  beneath  the  chordal  tube  so  formed,  so  that  this 
layer  becomes  a  continuous  sheet,  as  it  was  at  its  first  appearance. 


Fig.  47. — Transverse  Sections  through  Mole  Embryos,  showing  the  Formation 

of  the  Notochord. 
ec,  Ectoderm;  en,  endoderm;  m,  mesoderm;  nc.  notochord. — (Heape.) 


The  lumen  which  is  at  first  present  in  the  chordal  tube  is  soon 
obliterated  by  the  enlargement  of  the  cells  which  bound  it,  and 
these  cells  later  undergo  a  peculiar  transformation  whereby  the 
chordal  tube  is  converted  into  a  solid  elastic  rod  surrounded  by  a 
cuticular  sheath  secreted  by  the  cells.  The  notochord  lies  at  first 
immediately  beneath  the  median  line  of  the  medullary  groove,  be- 
tween the  ectoderm  and  the  endoderm,  and  has  on  either  side  of  it 
the  mesodermal  plates.  It  is  a  temporary  structure  of  which  only 
rudiments  persist  in  the  adult  condition  in  man,  but  it  is  a  structure 
characteristic  of  all  vertebrate  embryos  and  persists  to  a  more  or 
less  perfect  extent  in  many  of  the  fishes,  being  indeed  the  only  axial 


j6  THE    MESODERMIC    SOMITES 

skeleton  possessed  by  Amphioxus.  In  the  higher  vertebrates  it  is 
almost  completely  replaced  by  the  vertebral  column,  which  develops 
around  it  in  a  manner  to  be  described  later. 

The  Mesodermic  Somites. — Turning  now  to  the  middle 
germinal  layer,  it  will  be  found  that  in  it  also  important  changes  take 
place  during  the  early  stages  of  development.  The  probable  mode 
of  development  of  the  extra-embryonic  mesoderm  and  body-cavity 
has  already  been  described  (p.  67)  and  attention  may  now  be  directed 
toward  what  occurs  in  the  embryonic  mesoderm.  In  both  the 
Peters  embryo  and  the  embryo  v.H  described  by  von  Spee  this 
portion  of  the  mesoderm  is  represented  by  a  plate  of  cells  lying 
between  the  ectoderm  and  endoderm  and  becoming  continuous  at 
the  edges  of  the  embryonic  area  with  both  the  layer  which  surrounds 
the  yolk-sac  and,  through  the  mesoderm  of  the  belly-stalk,  with  the 
chorionic  mesoderm  (Fig.  37).  It  seems  probable,  since  there  is  in 
these  embryos  no  indication  as  yet  of  the  formation  of  the  chorda 
endoderm,  that  this  plate  of  mesoderm  corresponds  to  the  prostomial 
mesoderm  of  lower  forms.  In  older  embryos,  such  as  the  embryo 
Gle  of  Graf  Spee  and  the  younger  embryo  described  by  Eternod 
(Fig.  43),  the  mesoderm  no  longer  forms  a  continuous  sheet  extend- 
ing completely  across  the  embryonic  disk,  but  is  divided  into  two 
lateral  plates,  in  the  interval  between  which  the  ectoderm  of  the 
floor  of  the  medullary  groove  and  the  chorda  endoderm  are  in  close 
contact  (Fig.  48).  These  lateral  plates  represent  the  gastral  meso- 
derm, whose  origin  has  already  been  described  (p.  59),  and  which 
apparently  supplants  the  original  prostomial  mesoderm,  whose 
fate  in  the  human  embryo  is  at  present  unknown.  The  changes 
which  now  occur  have  not  as  yet  been  observed  in  the  human  embryo, 
though  they  probably  resemble  those  described  in  other  mammalian 
embryos,  and  the  phenomena  which  occur  in  the  sheep  may  serve 
to  illustrate  their  probable  nature. 

It  has  been  seen  that  in  the  stage  represented  by  the  Frassi 
embryo  a  plate  of  mesoderm  has  formed  on  either  side  of  the  chorda 
endoderm,  and  that  in  a  later  stage,  represented  by  the  Kromer 
embryo  Klb,  a  differentiation  occurs  in  these  plates  leading  to  the 


THE   MESODERMIC   SOMITES  77 

formation  of  mesodermic  somites.  These  make  their  appearance 
in  what  will  later  be  the  cervical  region  of  the  embryo  and  their 
formation  proceeds  backward  as  the  body  of  the  embryo  increases 
in  length.  A  longitudinal  groove  appears  on  the  dorsal  surface  of 
each  lateral  plate  of  mesoderm,  marking  off  the  more  median  thicker 
portion  from  the  lateral  parts  (Fig.  48),  which  from  this  stage 
onward  may  be  termed  the  ventral  mesoderm.  The  median  or  dorsal 
portions  then  become  divided  transversely  into  a  number  of  more 
or  less  cubical  masses  which  are  termed  the  protoverlebrce  or,  better, 


Fig.  48. — Transverse  Section  through  the  Second  Mesodermic  Somite  of  a 
Sheep  Embryo  3  mm.  Long. 
am,  Amnion;  en,  endoderm;  I,  intermediate  cell-mass;  mg,  medullary  groove;  ms, 
mesodermic  somite;  so,  somatic  and  sp,  splanchnic  layers  of  the  ventral  mesoderm. — 
(Bonnet.) 

mesodermic  somites  (Fig.  48,  ms).  The  cells  of  the  somites  and  of 
the  ventral  mesoderm,  are  at  first  stellate  in  form,  but  later  become 
more  spindle-shaped,  and  those  near  the  center  of  each  somite  and 
those  of  the  ventral  mesoderm  arrange  themselves  in  regular  layers 
so  as  to  enclose  cavities  which  appear  in  these  regions  (Fig.  48). 
Each  original  lateral  plate  of  gastral  mesoderm  thus  becomes 
divided  longitudinally  into  three  areas,  a  more  median  area  com- 
posed of  mesodermic  somites,  lateral  to  this  a  narrow  area  under- 
lying the  original  longitudinal  groove  which  separated  the  somite 
area  from  the  ventral  mesoderm  and  which  from  its  position  is 
termed  the  intermediate  cell-mass  (Fig.  48, 1) ,  and,  finally,  the  ventral 
mesoderm.     This  last  portion  is  now  divided  into  two  layers,  the 


78  THE    MESODERMIC    SOMITES 

dorsal  of  which  is  termed  the  somatic  mesoderm,  while  the  ventral  one 
is  known  as  the  splanchnic  mesoderm  (Fig.  48,  so  and  sp;  and  Fig.  49) , 
the  cavity  which  separates  these  two  layers  being  the  embryonic 
body-cavity  or  pleuroperitoneal  cavity  (coslom) ,  which  will  eventually 
give  rise  to  the  pleural,  pericardial  and  peritoneal  cavities  of  the  adult 
as  well  as  the  cavity  of  each  tunica  vaginalis  testis. 


Fig.  49. — Transverse  Section  of  an  Embryo  of  2.5  mm.  (See  Fig.  53)  showing 
on  either  side  of  the  medullary  canal  a  mesodermic  somite,  the  inter- 
MEDIATE Cell-mass,  and  the  Ventral  Mesoderm. — (vonLenhossek.) 

Beginning  in  the  neck  region,  the  formation  of  the  mesodermic 
somites  proceeds  posteriorly  until  finally  there  are  present  in  the 
human  embryo  thirty-eight  pairs  in  the  neck  and  trunk  regions  of 
the  body,  and,  in  addition,  a  certain  number  are  developed  in  what 
is  later  the  occipital  region  of  the  head.  Exactly  how  many  of  these 
occipital  somites  are  developed  is  not  known,  but  in  the  cow  four 
have  been  observed,  and  there  are  reasons  for  believing  that  the 
same  number  occurs  in  the  human  embryo. 

In  the  lower  vertebrates  a  number  of  cavities  arranged  in  pairs  occur 
in  the  more  anterior  portions  of  the  head  and  have  been  homologized  with 
mesodermic  somities.     Whether  this  homology  be  perfectly  correct  or  not, 


THE    MESODERMIC    SOMITES 


79 


these  head-cavities,  as  they  are  termed,  indicate  the  existence  of  a  division 
of  the  head  mesoderm  into  somites,  and  although  practically  nothing 
is  known  as  to  their  existence  in  the  human  embryo,  yet,  from  the  relations 
in  which  they  stand  to  the  cranial  nerves  and  musculature  in  the  lower 
forms,  there  is  reason  to  suppose  that  they  are  not  entirely  unrepresented 


\\'W^; — M 


.*$'$te$&\&  P 


1  • ; 

Vu 


1  -    -4  ; 


Fig.  50. — Transverse  Section  of  an  Embryo  of  4.25  mm.  at  the  Level  of  the  Arm 

Rudiment. 
A,  Axial  mesoderm  of  arm;  Am,  amnion;  il,  inner  lamella  of  myotome;  M,  myotome; 
me,  splanchnic  mesoderm;  ol,  outer  lamella  of  myotome;  Pn,  place  of  origin  of  pro- 
nephros;^ sclerotome;  S1,  defect  in  wall  of  myotome  due  to  separation  of  the  sclerotome; 
st,  stomach ;  Vu,  umbilical  vein. — (Kollmann.) 


The  mesodermic  somites  in  the  earliest  human  embryos  in 
which  they  have  been  observed  contain  a  completely  closed  cavity, 
and  this  is  true  of  the  majority  of  the  somites  in  such  a  form  as  the 
sheep.  In  the  four  first-formed  somites  in  this  species,  however, 
the  somite  cavity  is  at  first  continuous  with  the  pleuroperitoneal 


3o  THE  MESODERMIC   SOMITES 

cavity  and  only  later  becomes  separated  from  it,  and  in  lower  verte- 
brates this  continuity  of  the  somite  cavities  with  the  general  body- 
cavity  is  the  rule.  The  somite  cavities  are  consequently  to  be 
regarded  as  portions  of  the  general  pleuroperitoneal  cavity  which 
have  secondarily  been  separated  off.  They  are,  however,  of  but 
short  duration  and  early  become  filled  up  by  spindle-shaped  cells 
derived  from  the  walls  of  the  somites,  which  themselves  undergo  a 
differentiation  into  distinct  portions.  The  cells  of  that  portion  of  the 
wall  of  each  somite  which  is  opposite  the  notochord  become  spindle- 
shaped  and  grow  inward  toward  the  median  line  to  surround  the 
notochord  and  central  nervous  system,  and  give  rise  eventually  to 
the  lateral  half  of  the  body  of  a  vertebra  and  the  corresponding 
portion  of  a  vertebral  arch.  This  portion  of  the  somite  is  termed  a 
sclerotome  (Fig.  50,  S),  and  the  remainder  forms  a  muscle  plate  or 
myotome  (M)  which  is  destined  to  give  rise  to  a  portion  of  the  volun- 
tary musculature  of  the  body.  The  outer  wall  of  the  somite  has 
been  generally  believed  to  take  part  in  the  formation  of  the  cutis 
layer  of  the  integument  and  hence  has  been  termed  the  cutis  plate 
or  dermatome,  but  it  seems  probable  that  it  becomes  entirely  trans- 
formed into  muscular  tissue. 

The  intermediate  cell-mass  in  the  human  embryo,  as  in  lower 
forms,  partakes  of  the  transverse  divisions  which  separate  the  individ- 
ual mesodermic  somites.  From  one  portion  of  the  tissue  in  most  of 
the  somites  (Fig.  50,  Pri)  the  provisional  kidneys  or  Wolffian  bodies 
develop,  this  portion  of  each  mass  being  termed  a  nephrotome,  while 
the  remaining  portion  gives  rise  to  a  mass  of  cells  showing  no  tend- 
ency to  arrange  themselves  in  definite  layers  and  constituting  that 
form  of  mesoderm  which  has  been  termed  mesenchyme  (see  p.  61). 
These  mesenchymatous  masses  become  converted  into  connective 
tissues  and  blood-vessels. 

The  ventral  mesoderm  in  the  neck  and  trunk  regions  never 
becomes  divided  transversely  into  segments  corresponding  to  the 
mesodermic  somites,  differing  in  this  respect  from  the  other  portions 
of  the  gastral  mesoderm.  In  the  head,  however,  that  portion 
of  the  middle  layer  which  corresponds  to  the  ventral  mesoderm  of 


THE   MESODERMIC    SOMITES  8 1 

the  trunk  does  undergo  a  division  into  segments  in  connection  with 
the  development  of  the  branchial  arches  and  clefts  (see  p.  90).  A 
consideration  of  these  segments,  which  are  known  as  the  branchio- 
meres,  may  conveniently  be  postponed  until  the  chapters  dealing 
with  the  development  of  the  cranial  muscles  and  nerves,  and  in  what 
follows  here  attention  will  be  confined  to  what  occurs  in  the  ventral 
mesoderm  of  the  neck  and  trunk. 

Its  splanchnic  layer  (Fig.  51,  vm),  applies  itself  closely  to  the 
endodermal  digestive  tract,  which  is  constricted  off  from  the  dorsal 
portion  of  the  yolk-sac,  and  becomes  converted  into  mesenchyme 
out  of  which  the  muscular  coats  of  the  digestive  tract  develop. 
The  cells  which  line  the  pleuroperitoneal  cavity,  however,  retain 
their  arrangement  in  a  layer  and  form  a  part  of  the  serous  lining  of 
the  peritoneal  and  other  serous  cavities,  the  remainder  of  the  lining 
being  formed  by  the  corresponding  cells  of  the  somatic  layer;  and 
in  the  abdominal  region  the  superficial  cells,  situated  near  the  line 
where  the  splanchnic  layer  passes  into  the  somatic,  and  in  close 
proximity  to  the  nephrotome  of  the  intermediate  cell-mass,  become 
columnar  in  shape  and  are  converted  into  reproductive  cells. 

The  somatic  layer,  if  traced  peripherally,  becomes  continuous 
at  the  sides  with  the  layer  of  mesoderm  which  lines  the  outer  surface 
of  the  amnion  (Fig.  50)  and  posteriorly  with  the  mesoderm  of  the 
belly-stalk.  That  portion  of  it  which  lies  within  the  body  of  the 
embryo,  in  addition  to  giving  rise  to  the  serous  lining  of  the  parietal 
layer  of  the  pleuroperitoneum,  becomes  converted  into  mesenchyme, 
which  for  a  considerable  length  of  time  is  clearly  differentiated  into 
two  zones,  a  more  compact  dorsal  one  which  may  be  termed  the 
somatic  layer  proper,  and  a  thinner,  more  ventral  vascular  zone 
which  is  termed  the  membrana  reuniens  (Fig.  51).  In  the  earlier 
stages  the  somatic  layer  proper  does  not  extend  ventrally  beyond 
the  line  which  passes  through  the  limb  buds  and  it  grows  out  into 
these  buds  to  form  an  axial  core  for  them,  in  which  later  the  skeleton 
of  the  limb  forms.  The  remainder  of  the  mesoderm  lining  the  sides 
and  ventral  portions  of  the  body-wall  is  at  first  formed  from  the 
membrana  reuniens,  but  as  development  proceeds  the  somatic 
6 


82 


THE   MESODERMIC    SOMITES 


layer  gradually  extends  more  ventrally  and  displaces,  or,  more 
properly  speaking,  assimilates  into  itself,  the  membrana  reuniens 
until  finally  the  latter  has  completely  disappeared. 

It  is  to  be  noted  that  no  part  of  the  voluntary  musculature 
of  the  lateral  and  ventral  walls  of  the  neck  and  trunk  is  derived 
from  the  somatic  layer;  it  is  formed  entirely  from  the  myotomes 
which  gradually  extend  ventrally  (Fig.  51)  and  finally  come  into 
contact  with  their  fellows  of  the  opposite  side  in  the  mid-ventral  line. 


Fig.  51. — Diagrams  Illustrating  the  History  of  the  Gastral  Mesoderm. 

dM,  dorsal  portion  of  myotome;  gr,  genital  ridge;  I,  intestine;  M,  myotome,  mr, 
membrana  reuniens;  N,  nervous  system;  SC,  sclerotome;  Sm,  somatic  mesoderm; 
vm,  splanchnic  mesoderm;  vM,  ventral  portion  of  myotome;  Wd,  Wolffian  duct. 

Whether  the  voluntary  musculature  of  the  limbs  is  also  derived 
from  the  myotomes  is  at  present  doubtful.  It  has  been  very  generally 
believed  that  the  myotomes  in  their  growth  ventrally  sent  prolon- 
gations into  the  limb  buds  which  invested  the  axial  core  of  mesen- 
chyme and  eventually  gave  rise  to  the  voluntary  muscles.  The 
actual  existence  of  the  prolongations  of  the  myotomes  and  their 
conversion  into  the  limb  musculature  has,  however,  not  yet  been 
observed  and  it  is  quite  possible  that  the  limb  musculature  may  be 
derived  from  the  axial  core  of  somatic  mesoderm  from  which  the 
limb  skeleton  develops. 

The  appearance  of  the  mesodermic  somites  is  an  important 


THE    MESODERMIC    SOMITES  8$ 

phenomenon  in  the  development  of  the  embryo,  since  it  influences 
fundamentally  the  future  structure  of  the  organism.  If  each  pair 
of  mesodermic  somites  be  regarded  as  a  structural  unit  and  termed 
a  metamere  or  segment,  then  it  may  be  said  that  the  body  is  com- 
posed of  a  series  of  metameres,  each  more  or  less  closely  resembling 
its  fellows,  and  succeeding  one  another  at  regular  intervals.  Each 
somite  differentiates,  as  has  been  stated,  into  a  sclerotome  and  a 
myotome,  and,  accordingly,  there  will  primarily  be  as  many  verte- 
bra? and  muscle  segments  as  there  are  mesodermic  somites,  or,  in 
other  words,  the  axial  skeleton  and  the  voluntary  muscles  of  the 
trunk  are  primarily  metameric.  Nor  is  this  all.  Since  each 
metamere  is  a  distinct  unit,  it  must  possess  its  own  supply  of  nutri- 
tion, and  hence  the  primary  arrangement  of  the  blood-vessels  is  also 
metameric,  a  branch  passing  off  on  either  side  from  the  main  longi- 
tudinal arteries  and  veins  to  each  metamere.  And,  further,  each 
pair  of  muscle  segments  receives  its  own  nerves,  so  that  the  arrange- 
ment of  the  nerves,  again,  is  distinctly  metameric. 

It  is  to  be  noted  that  this  metamerism  is  essentially  resident  in 
the  dorsal  mesoderm,  the  segmentation  shown  by  structures  derived 
from  other  embryonic  tissues  being  secondary  and  associated  with 
the  relations  of  these  structures  to  the  mesodermic  somites.  The 
metamerism  is  most  distinct  in  the  neck  and  trunk  regions,  and  at 
first  only  in  the  dorsal  portions  of  these  regions,  the  ventral  portions 
showing  metamerism  only  after  the  extension  into  them  of  the  myo- 
tomes. But  there  is  clear  evidence  that  the  arrangement  extends 
also  into  the  head,  and  that  a  portion  of  its  mesoderm  is  to  be  regarded 
as  composed  of  metameres.  It  has  been  seen  that  in  the  noto- 
chordal  region  of  the  head  of  lower  vertebrates  mesodermic  somites 
are  present,  while  anteriorly  in  the  prechordal  region  there  are  head- 
cavities  which  resemble  closely  the  mesodermic  somites,  and  are 
probably  directly  comparable  to  the  somites  of  the  trunk.  There  is 
reason,  therefore,  for  believing  that  the  fundamental  arrangement 
of  the  dorsal  mesoderm  in  all  parts  of  the  body  is  metameric,  but 
though  this  arrangement  is  clearly  defined  in  early  embryos,  it 
loses  distinctness  in  later  periods  of  development.     But  even  in  the 


84  LITERATURE 

adult  the  original  metamerism  is  clearly  indicated  in  the  arrange- 
ment of  the  nerves  and  of  parts  of  the  axial  skeleton,  and  careful 
study  frequently  reveals  indications  of  it  in  highly  modified  muscles 
and  blood-vessels. 

In  the  head  the  development  of  the  branchial  arches  and  clefts 
produces  a  series  of  parts  presenting  many  of  the  peculiarities  of 
metameres,  and,  indeed,  it  has  been  a  very  general  custom  to  regard 
them  as  expressions  of  the  general  metamerism  which  prevails 
throughout  the  body.  It  is  to  be  noted,  however,  that  they  are  pro- 
duced by  the  segmentation  of  the  ventral  mesoderm,  a  structure 
which  in  the  neck  and  trunk  regions  does  not  share  in  the  general 
metamerism,  and,  furthermore,  recent  observations  on  the  cranial 
nerves  seem  to  indicate  that  these  branchiomeres  cannot  be  regarded 
as  portions  of  the  head  metameres  or  even  as  structures  compara- 
ble to  these.  They  represent,  more  probably,  a  second  metamerism 
superposed  upon  the  more  general  one,  or,  indeed,  possibly  more 
primitive  than  it,  but  whose  relations  can  only  be  properly  under- 
stood in  connection  with  a  study  of  the  cranial  nerves. 

LITERATURE. 

In  addition  to  many  of  the  papers  cited  in  the  list  at  the  close  of  Chapter  II,  the 
following  may  be  mentioned: 
C.  R.  Bardeen:  "  The  Development  of  the  Musculature  of  the  Body  Wall  in  the  Pig, 

etc.,"  Johns  Hopkins  Hosp.  Rep.,  ix,  1900. 
T.  H.  Bryce  and  J.  H.  Teacher:  "  Contributions  to  the  Study  of  the  Early  Develop- 
ment and  Imbedding  of  the  Human  Ovum,"  Glasgow,  1908. 
A.  C.  F.  Eternod:  "Communication  sur  un  ceuf  humain  avec   embryon    excessive- 

ment  jeune,"  Arch.  Ital.  de  Biologie,  xxn,  1895. 
A.  C.  F.  Eternod:  "II  y  a  un  canal  notochordal  dans  l'embryon  humain,"  Anat. 

Anzeiger,  xvi,  1899. 
Fetzer:  "Ueber  ein  durch  Operation  gewonnenes  menschliches  Ei  das  in  seiner 

Entwickelung  etwa  dem   Peterssehen  Ei  entspricht,"    Verh.    Anat.  Gesellschaft, 

xxiv,  1910. 
L.  Frassi:  "Weitere  Ergebnisse  des  Studiums  eines  jungen  menschlichen   Eies  in 

situ,"  Arch.f.  mikr.  Anat.,  lxxi,  1908. 
W.  Heape:  "The  Development  of  the  Mole  (Talpa  Europaea),"  Quarterly  Journ. 

Microsc.  Science,  xxvn,  1887. 
M.  Herzog:  "A  Contribution  to  our  Knowledge  of  the  Earliest  Known  Stages  of 

Placentation  and  Embryonic  Development  in  Man,"  Amer.  Journ.  Anat.,  ix,  1909. 


LITERATURE  85 

F.  Keibel:  "Zur  Entwickelungsgeschichte  der  Chorda  bei  Saugern    (Meerschwein- 

chen  und  Kaninchen),"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1889. 
S.  Kaestner:  "Ueber  die  Bildung  von  animalen  Muskelfasern  aus  dem   Urwirbel," 

Arch,  filr  Anat.  und  Phys.,  Anat.  Abth.,  Suppl.,  1890. 
J.  Kollmann:  "Die  Rumpfsegmente  menschlicher  Embryonen  von  13  bis  35  Unvir- 

beln,"  Archiv  filr  Anat.  und  Physiol.,  Anat.  Abth.,  1891. 
H.  Peters:  "Ueber  die  Einbettung  des  menschlichen  Eies  und   das  friiheste  bisher 

bekannte  menschliche  Placentarstadium,"  Leipzig  und  Wien,  1899. 
F.  Graf  von  Spee:  "  Beobachtungen    an    einer    menschlichen    Keimscheibe    mit 

offener  Medullarrinne  und  Canalis  neurentericus,"  Arch.f.  Anat.  u.  Phys.,  Anat. 

Abth.,  1889. 
F.  Graf  von  Spee:  "Ueber    friihe    Entwicklungsstufen    des    menschlichen    Eies," 

Arch.f.  Anat.  u.  Phys.,  Anat.  Abth.,  1896. 
H.  Strahl  and  R.  Beneke:  "Ein  junger  menschlicher  Embryo,"  Wiesbaden,  1910. 
J.  W.  VAN  Wijhe:  "Ueber  die  Mesodermsegmente  des  Rumpfes  und  die  Entwick- 

lung  des  Excretionsystems  bei  Selachiern,"  Archiv  fur  mikrosk.  Anat.,  xxxin, 

1889. 
K.  W.  Zimmermann:  "  Ueber  Kopfhohlenrudimente  beim  Menschen,"   Archiv   filr 

mikrosk.  Anat.,  liii,  1898. 


CHAPTER  IV. 

THE  DEVELOPMENT  OF  THE  EXTERNAL  FORM  OF  THE 
HUMAN  EMBRYO. 

In  the  preceding  chapter  descriptions  have  been  given  of  human 
embryos  representing  the  earlier  known  stages  and  the  development 
of  the  general  form  of  the  human  embryo  has  been  traced  up  to  the 
time  when  the  mesodermic  somites  have  made  their  appearance. 
It  will  now  be  convenient  to  continue  the  history  of  the  general 
development  up  to  the  stage  when  the  embryo  becomes  a  fetus. 

In  the  earlier  stages,  that  is  to  say  up  to  that  represented  by  the 
Eternod  embryo  (Fig.  43),  the  embryonic  disk  may  be  described  as 
floating  upon  the  surface  of  the  yolk-sac,  and  while  this  description 
still  holds  good  for  the  Eternod  embryo  a  distinct  groove  may  be  seen 
in  that  embryo  between  the  peripheral  portions  of  the  embryonic 
disk  and  the  upper  part  of  the  sac.  This  groove  marks  the  beginning 
of  the  separation  or  constriction  of  the  embryo  from  the  yolk-sac, 
the  result  of  which  is  the  transformation  of  the  discoidal  embryonic 
portion  of  the  embryonic  disk  into  a  cylindrical  structure.  Pri- 
marily this  depends  upon  the  deepening  of  the  furrow  which  sur- 
rounds the  embryonic  area,  the  edges  of  this  area  being  thus  bent  in 
on  all  sides  toward  the  yolk-sac.  This  bending  in  proceeds  most 
rapidly  at  the  anterior  end  of  the  body,  as  shown  in  the  diagrams 
(Fig.  52),  and  less  rapidly  at  the  posterior  end  where  the  belly- 
stalk  is  situated,  and  produces  a  constriction  of  the  yolk-sac,  the 
portion  of  this  structure  nearest  the  embryonic  disk  becoming  en- 
closed within  the  body  of  the  embryo  to  form  the  digestive  tract, 
while  the  remainder  is  converted  into  a  pedicle-like  portion,  the 
yolk-stalk, '  at  the  extremity  of  which  is  the  yolk-vesicle.  The 
further  continuance  of  the  folding  in  of  the  edges  of  the  embryonic 
area  leads  to  an  almost  complete  closing  in  of  the  embryonic  ccelom 

86 


DEVELOPMENT  OF  EXTERNAL  FORM 


87 


and  reduces  the  opening  through  which  the  yolk-stalk  and  belly- 
stalk  communicate  with  the  embryonic  tissues  to  a  small  area  known 
as  the  umbilicus. 

In  the  Kromer  embryo  Klb  (Fig.  44)  this  separation  of  the  em- 
bryo proper  from  the  yolk-sac  has  proceeded  to  such  an  extent  that 
both  extremities  of  the  embryonic  disk  are  free  from  the  yolk-sac, 
and  the  anterior  extremity  is  bent  ventrally  almost  at  a  right  angle  to 


Fig.  52. — Diagrams  Illustrating  the  Constriction  of  the  Embryo  from  the 

Yolk-sac. 
A  and  C  are  longitudinal,  and  B  and  D  transverse  sections.     B  is  drawn  to  a  larger  scale 

than  the  other  figures. 


the  rest  of  the  disk,  producing  what  is  termed  the  vertex  bend,  a 
feature  characteristic  of  all  later  embryos.  The  marked  develop- 
ment in  this  embryo  of  the  medullary  folds  and  the  occurrence  of 
mesodermic  somites  have  already  been  mentioned  (p.  72). 

Somewhat  more  advanced  is  the  Bulle  embryo  described  by 
Kollmann  and  shown  from  the  side  and  dorsally  in  Fig.  53,  the 
greater  part  of  the  yolk-sac  having  been  removed  as  well  as  the  most 
of  the  amnion.  The  embryo  measured  about  2.5  mm.  in  length  and 
showed  a  considerable  increase  in  the  number  of  mesodermic 
somites,  there  being  about  fourteen  of  them  on  either  side.    Pos- 


88  DEVELOPMENT  OF  EXTERNAL  FORM 

teriorly  the  medullary  groove  has  become  converted  into  a  medul- 
lary canal  by  the  medullary  folds  meeting  over  it  and  fusing,  but 
anteriorly  it  is  still  open.     The  vertex  bend  is  well  marked  and 


am-\ 


Om 


M^'LrX.     j 


Y. 


-am 


Fig.  53. — Embryo  2.5  mm.  Long. 

om,  Amnion;  B,  belly-stalk;  h,  heart;  M,  closed,  and  M',  still  open  portions  of  the 

medullary  groove;  Om,  vitelline  vein;  OS,  oral  fossa;  Y,  yolk-sac. — {Kallmann.) 


immediately  behind  the  tip  of  the  head,  on  the  ventral  surface  of  the 
body,  there  may  be  seen  a  well-marked  depression,  the  oral  fossa, 
between  which  and  the  anterior  surface  of  the  yolk-sac  is  a  rounded 


DKVELOPMENT    OF    EXTERNAL    FORM 


89 


Fig.  54. — Embryo  Lr,  4.2  mm.  Long. 

am,  Amnion;  au,  auditory  capsule;  B,  belly-stalk;  h,  heart;  LI,  lower,  and  Ul,  upper 

limb;  Y,  yolk-sac. — (His.) 


90  DEVELOPMENT  OF  EXTERNAL  FORM 

elevation  due  to  the  formation  of  the  heart.  Attention  may  be 
called  to  the  fact  that  the  position  of  this  organ  is  far  forward  of  that 
which  it  will  eventually  occupy,  so  that  it  must  undergo  a  marked 
retrogression  during  later  development. 

As  an  example  of  a  later  stage. of  development  the  embryo Lr  of 
His,  measuring  4.2  mm.  in  length,  may  be  taken  (Fig.  54).  In  this 
the  constriction  of  the  yolk-sac  has  progressed  so  far  that  its  proxi- 
mal portion  may  now  be  spoken  of  as  the  yolk-stalk.  The  meso- 
dermic  somites  have  undergone  a  further  increase  and  have  almost 
reached  their  final  number,  the  vertex  bend  has  become  still  more 
pronounced  and  the  medullary  groove,  throughout  its  entire  length, 
has  been  converted  into  the  medullary  canal,  which,  anteriorly,  shows 
distinct  enlargements  and  constrictions  which  foreshadow  various 
portions  of  the  future  brain.  The  auditory  organ,  which  made  its 
appearance  in  earlier  stages,  has  now  become  quite  distinct,  and  a 
lateral  bulging  of  the  most  anterior  portion  of  the  head  indicates  the 
position  of  the  future  eye. 

In  addition  certain  other  important  features  have  now  appeared. 
Thus,  about  opposite  the  head  a  second  bend,  the  nape  bend,  is 
becoming  visible  on  the  dorsal  surface  of  the  body  and  toward  the 
posterior  end  a  distinct  sacral  bend  is  evident.  Secondly,  a  little 
posterior  to  the  level  of  the  nape  bend  a  slight  elevation  is  to  be  seen 
on  the  side  of  the  body;  this  is  the  limb  bud  for  the  upper  limb  and 
a  corresponding,  though  smaller,  elevation  in  the  region  of  the  sacral 
bend  represents  the  lower  limb. 

Thirdly,  three  grooves  having  a  dorso-ventral  direction  have 
appeared  on  the  sides  of  what  will  be  the  future  pharyngeal  region. 
These  are  representatives  of  a  series  of  branchial  clefts,  structures 
that  are  of  great  morphological  importance  in  the  further  develop- 
ment inasmuch  as  they  determine  to  a  large  extent  the  arrangement 
of  various  organs  of  the  head  region.  They  represent  the  clefts 
which  exist  in  the  walls  of  the  pharynx  in  fishes,  through  which 
water,  taken  in  at  the  mouth,  passes  to  the  exterior,  bathing  on  its 
way  the  gill  filaments  attached  to  the  bars  or  arches,  as  they  are 
termed,  which  separate  successive  clefts.     Hence  the  name  "bran- 


DEVELOPMENT  OF  EXTERNAL  FORM 


91 


Fig.  55. — Floor  of  the  Pharynx 
of  Embryo  B,  7  mm.  Long. 
Ep,  Epiglottis;  Sp,  sinus  prsecervi- 
calis;  t1,  tuberculum  impar;  t2, 
posterior  portions  of  the  tongue; 
I,  II,  III,  and  IV,  branchial  arches. 
—(His.) 


chial"  which  is  applied  to  them,  though  in  the  mammals  they  never 
have  respiratory  functions  to  perform,  but,  appearing,  persist  for 
a  time  and  then  either  disappear  or  are  applied  to  some  entirely  dif- 
ferent purpose.  Indeed,  in  man  they  are  never  really  clefts  but 
merely  grooves,  and  corresponding  to 
each  groove  in  the  ectoderm  there  is 
also  one  in  the  subjacent  endoderm 
of  what  will  eventually  be  the  pharyn- 
geal region  of  the  digestive  tract,  so 
that  in  the  region  of  each  cleft  the 
ectoderm  and  endoderm  are  in  close 
relation,  being  separated  only  by  a 
very  thin  layer  of  mesoderm.  In 
the  intervals  between  successive  clefts 
a  more  considerable  amount  of  meso- 
derm is  present  (Fig.  55). 

In  the  human  embryo  four  clefts 
and    five   branchial   arches   develop 
on  each  side  of  the  body,  the  last  arch  lying  posteriorly  to  the  fourth 
cleft  and  not  being  very  sharply  denned  along  its  posterior  margin. 

As  just  stated,  the  clefts  are  normally  merely  grooves,  and  in  later 
development  either  disappear  or  are  converted  into  special  structures. 
Occasionally,  however,  a  cleft  may  persist  and  the  thin  membrane  which 
forms  its  floor  may  become  perforated  so  that  an  opening  from  the  exterior 
into  the  pharynx  occurs  at  the  side  of  the  neck,  forming  what  is  termed  a 
branchial  fistula.  Such  an  abnormality  is  most  frequently  developed 
from  the  lower  (ventral)  part  of  the  first  cleft;  normally  this  disappears, 
the  upper  portion  of  the  cleft  persisting,  however,  to  form  the  external 
auditory  meatus  and  tympanic  cavity. 

A  further  stage  in  the  differentiation  of  these  clefts  and  arches 
is  shown  by  the  embryo  represented  in  Fig.  56.  The  nape  bend 
has  now  increased  to  such  an  extent  that  the  whole  anterior  part  of 
the  body  is  bent  at  a  right  angle  to  the  middle  part  and  the  entire 
embryo  is  coiled  in  a  spiral  manner.  The  limb  buds  are  much  more 
distinct  than  in  the  previous  stage  and  four  branchial  arches  are 
now  present;  the  second  and  third  have  become  more  defined  and 


92  DEVELOPMENT  OF  EXTERNAL  FORM 

a  strong  process  has  developed  from  the  dorsal  part  of  the  anterior 
border  of  the  first  one,  which  has  thus  become  somewhat  <3 -shaped. 
The  anterior  limb  of  each  V  is  destined  to  give  rise  to  the  upper  jaw, 
and  hence  is  known  as  the  maxillary  process,  while  the  posterior 
limb  represents  the  future  lower  jaw  and  is  termed  the  mandibular 
process. 


M— — — I— 

Fig.  56.— Embryo  Backer,  7.3  mm.  in  Length.  X5. — (Keibefand  Ehe.) 

In  the  stage  represented  by  this  embryo  the  closing  in  of  the 
embryonic  ccelom  has  progressed  to  such  a  degree  that  only  a  small 
opening  is  left  in  the  ventral  body-wall  of  the  embryo  through  which 
the  yolk-stalk  and  its  accompanying  vessels  and  the  belly-stalk  pass. 
Indeed  the  margins  of  the  umbilicus  may  have  begun  to  be  pro- 
longed outward  over  these  structures,  enclosing  them  in  a  cylindrical 
investment,  the  first  stage  of  what  will  later  be  the  umbilical  cord 
being  thus  established. 


DEVELOPMENT  OF  EXTERNAL  FORM 


93 


Leaving  aside  for  the  present  all  consideration  of  the  further 
development  of  the  limbs  and  branchial  arches,  the  further  evolution 
of  the  general  form  of  the  body  may  be  rapidly  sketched.  In  an 
embryo  (Fig.  57)  from  Ruge's  collection,  described  and  figured  by 
His  and  measuring  9.1  mm.   in  length,*  the  prolongation  of  the 


_W 


L-LI 


Fig.  57. — Embryo  9.1  mm.  Long. 
LI,  Lower  limb;  U,  umbilical  cord;  Ul,  upper  limb;  Y,  yolk-sac. — (His.) 

margins  of  the  umbilicus  has  increased  until  more  than  half  the 
yolk-stalk  has  become  enclosed  within  the  umbilical  cord.  The 
nape  and  sacral  bends  are  still  very  pronounced,  although  the  embryo 
is  beginning  to  straighten  out  and  is  not  quite  so  much  coiled  as  in 
the  preceding  stage.     At  the  posterior  end  of  the  body  there  has 

*  This  measurement  is  taken  in  a  straight  line  from  the  most  anterior  portion  of  the 
nape  bend  to  the  middle  point  of  the  sacral  bend  and  does  not  follow  the  curvature 
of  the  embryo.  It  may  be  spoken  of  as  the  nape-rump  length  and  is  convenient  for  use 
during  the  stages  when  the  embryo  is  coiled  upon  itself. 


94  DEVELOPMENT  OF  EXTERNAL  FORM 

developed  a  rather  abruptly  conical  tail  filament,  in  the  place  of  the 
blunt  and  gradually  tapering  termination  seen  in  earlier  stages, 
and  a  well-marked  rotundity  of  the  abdomen,  due  to  the  rapidly 
increasing  size  of  the  liver,  begins  to  become  evident. 

In  later  stages  the  enclosure  of  the  yolk-  and  belly-stalks  within 
the  umbilical  cord  proceeds  until  finally  the  cord  is  complete  through 
the  entire  interval  between  the  embryo  and  the  wall  of  the  ovum. 
At  the  same  time  the  straightening  out  of  the  embryo  continues,  as 
may  be  seen  in  Fig.  58  representing  the  embryo  xlv  (Br2)  of  His, 
which  shows  also,  both  in  front  of  and  behind  the  neck  bend,  a 


Fig.  58.' — Embryo  B  r2,  13.6  mm.  Long. — (His.) 

distinct  depression,  the  more  anterior  being  the  occipital  and  the  more 
posterior  the  nape  depression;  both  these  depressions  are  the  indica- 
tions of  changes  taking  place  in  the  central  nervous  system.  The 
tail  filament  has  become  more  marked,  and  in  the  head  region  a  slight 
ridge  surrounding  the  eyeball  and  marking  out  the  conjunctival  area 
has  appeared;  a  depression  anterior  to  the  nasal  fossae  marks  off  the 
nose  from  the  forehead;  and  the  external  ear,  whose  development 
will  be  considered  later  on,  has  become  quite  distinct.  This  embryo 
had  a  nape-rump  length  of  13.6  mm. 


DEVELOPMENT  OF  EXTERNAL  FORM 


95 


In  the  embryos  xxxv  (S2)  and  xcix  (L3)  (Fig.  59,  A  and  B)  of 
His'  collection  the  straightening  out  of  the  nape  bend  is  proceeding, 
and  indeed  is  almost  completed  in  embryo  xcix,  which  begins  to 
resemble  closely  the  fully  formed  fetus.  The  tail  filament,  some- 
what reduced  in  size,  still  persists  and  the  rotundity  of  the  abdomen 
continues  to  be  well  marked.  The  neck  region  is  beginning  to  be 
distinguishable  in  embryo  S2  and  in  embryo  L3  the  eyelids  have 
appeared  as  slight  folds  surrounding  the  conjunctival  area.     The 


Fig.  59. — A,  Embryo  S2,  15  mm.  Long  (showing  Ectopia  of  the  Heart);  B,  Embryo 
L3,  17.5  mm.  Long. — (His.) 

nose  and  forehead  are  clearly  defined  by  the  greater  development 
of  the  nasal  groove  and  the  nose  has  also  become  raised  above  the 
general  surface  of  the  face,  while  the  external  ear  has  almost  acquired 
its  final  fetal  form.  These  embryos  measure  respectively  about 
15  and  17.5  mm.  in  length.* 

Finally,    an   embryo — again   one   of   those   described   by    His, 


*  The   embryo  S2  presents   a   slight   abnormality  [in   the  great   projection  of  the 
heart,  but  otherwise  it  appears  to  be  normal. 


96 


DEVELOPMENT  OF  EXTERNAL  EORM 


namely,  his  lxxvti  (Wt),  having  a  length  of  23  mm. — may  be 
figured  (Fig.  60)  as  representing  the  practical  acquisition  of  the 
fetal  form.  This  embryo  dates  from  about  the  end  of  the  second 
month  of  pregnancy,  and  from  this  period  onward  it  is  proper  to 
use  the  term  fetus  rather  than  that  of  embryo.     The  changes  which 


Fig.  60. — Embryo  Wt,  23  mm.  Long. — (His.) 


have  been  described  in  preceding  stages  are  now  complete  and  it 
remains  only  to  be  mentioned  that  the  caudal  filament,  which  is  still 
prominent,  gradually  disappears  in  later  stages,  becoming,  as  it 
were,  submerged  and  concealed  beneath  adjacent  parts  by  the 
development  of  the  buttocks.  The  incompleteness  of  the  develop- 
ment of  these  regions  in  embryo  Wt  is  manifest,  not  only  from  the 


DEVELOPMENT    OF    THE   BRANCHIAL  ARCHES 


97 


projection  of  the  tail  filament,  but  also  from  the  external  genitalia 
being  still  largely  visible  in  a  side  view  of  the  embryo,  a  condition 
which  will  disappear  in  later  stages. 

The  Later  Development  of  the  Branchial  Arches,  and  the 
Development  of  the  Face. — In  the  embryo  shown  in  Fig.  56,  the 
four  branchial  clefts  and  five  arches  which  develop  in  the  human 
embryo  are  visible  in  surface  views,  but  in  the  Ruge  embryo  (Fig.  57) 
it  will  be  noticed  that  only  the  first  two  arches,  the  first  with  a  well- 
developed  maxillary  process,  and  the  cleft  separating  them  can  be 


Fig.  61. — Head  of  Embryo  of  6.9  mm. 
na,  Nasal  pit;  ps,  precervical  sinus.— (His.) 

distinguished.  This  is  due  to  a  sinking  inward  of  the  region  occu- 
pied by  the  three  posterior  arches  so  that  a  triangular  depression, 
the  sinus  pracervicalis,  is  formed  on  each  side  of  what  will  later 
become  the  anterior  part  of  the  neck  region.  This  is  well  shown  in 
an  embryo  (Br3)  described  by  His  which  measured  6.9  mm.  in 
length  and  of  which  the  anterior  portion  is  shown  in  Fig.  61.  The 
anterior  boundary  of  the  sinus  {ps)  is  formed  by  the  posterior  edge 
7 


98         DEVELOPMENT  OF  THE  BRANCHIAL  ARCHES 

of  the  second  arch  and  its  posterior  boundary  by  the  thoracic  wall, 
and  in  later  stages  these  two  boundaries  gradually  approach  one 
another  so  as  first  of  all  to  diminish  the  opening  into  the  sinus  and 
later  to  completely  obliterate  it  by  fusing  together,  the  sinus  thus 
becoming  converted  into  a  completely  closed  cavity  whose  floor  is 
formed  by  the  ectoderm  covering  the  three  posterior  arches  and  the 
clefts  separating  these.  This  cavity  eventually  undergoes  degen- 
eration, no  traces  of  it  occurring  normally  in  the  adult,  although 


Fig.  62. — Face  of  Embryo  of  8  mm. 
mxp,  Maxillary  process;  np,  nasal  pit;  os,  oral  fossa;  pg,  processus  globularis.— (His.) 

certain  cysts  occasionally  observed  in  the  sides  of  the  neck  may 
represent  persisting  portions  of  it. 

A  somewhat  similar  process  results  in  the  closure  of  the  ventral 
portion  of  the  first  cleft,*  a  fold  growing  backward  from  the  posterior 
edge  of  the  first  arch  and  fusing  with  the  ventral  part  of  the  anterior 
border  of  the  second  arch.  The  upper  part  of  the  cleft  persists, 
however,  and,  as  already  stated,  forms  the  external  auditory  meatus, 
the  pinna  of  the  ear  being  developed  from  the  adjacent  parts  of 
the  first  and  second  arches  (Figs.  58  and  59). 

*  See  page  91,  small  type. 


DEVELOPMENT    OF    THE    FACE 


99 


The  region  immediately  in  front  of  the  first  arch  is  occupied  by 
a  rather  deep  depression,  the  oral  fossa,  whose  early  development 
has  already  been  noticed.  In  an  embryo  measuring  8  mm.  in 
length  (Fig.  62)  the  fossa  (os)  has  assumed  a  somewhat  irregular 
quadrilateral  form.  Its  posterior  boundary  is  formed  by  the 
mandibular  processes  of  the  first  arch,  while  laterally  it  is  bounded 
by  the  maxillary  processes  (mxp)  and  anteriorly  by  the  free  edge  of 
a  median  plate,  termed  the  nasal  process,  which  on  either  side  of  the 


Fig.  63. — Face  of  Embryo  after  the  Completion  of  the  Upper  Jaw. — (His.) 

median  line  is  elevated  to  form  a  marked  protuberance,  the  processus 
globular  is  (pg).  The  ventral  ends  of  the  maxillary  processes  are 
widely  separated,  the  nasal  process  and  the  processus  globulares 
intervening  between  them,  and  they  are  also  separated  from  the 
globular  processes  by  a  deep  and  rather  wide  groove  which  anteriorly 
opens  into  a  circular  depression,  the  nasal  pit  (np). 


IOO  DEVELOPMENT    OF    THE   LIMBS 

Later  on  the  maxillary  and  globular  processes  unite,  obliterating 
the  groove  and  cutting  off  the  nasal  pits — which  have  by  this  time 
deepened  to  form  the  nasal  fossae — from  direct  communication 
with  the  mouth,  with  which,  however,  they  later  make  new  com- 
munications behind  the  maxillary  processes,  an  indication  of  the 
anterior  and  posterior  nares  being  thus  produced. 

Occasionally  the  maxillary  and  globular  processes  fail  to  unite  on  one 
or  both  sides,  producing  a  condition  popularly  known  as  "harelip." 

At  the  time  when  this  fusion  occurs  the  nasal  fossa?  are  widely 
separated  by  the  broad  nasal  process  (Fig.  63),  but  during  later 
development  this  process  narrows  to  form  the  nasal  septum  and  is 
gradually  elevated  above  the  general  surface  of  the  face  as  shown 
in  Figs.  58-60.  By  the  narrowing  of  the  nasal  process  the  globular 
processes  are  brought  nearer  together  and  form  the  portions  of  the 
upper  jaw  immediately  on  each  side  of  the  median  line,  the  rest 
of  the  jaw  being  formed  by  the  maxillary  processes.  In  the  mean- 
time a  furrow  has  appeared  upon  the  mandibular  process,  running 
parallel  with  its  borders  (Fig.  59);  the  portion  of  the  process  in  front 
of  this  furrow  gives  rise  to  the  lower  lip  and  is  known  as  the  lip 
ridge,  while  the  portion  behind  the  furrow  becomes  the  lower  jaw 
proper  and  is  termed  the  chin  ridge. 

The  Development  of  the  Limbs. — As  has  been  already  pointed 
out,  the  limbs  make  their  appearance  in  an  embryo  measuring  about 
4  mm.  in  length  (Fig.  54)  and  are  at  first  bud-like  in  form.  As  they 
increase  in  length  they  at  first  have  their  long  axes  directed  parallel 
to  the  longitudinal  axis  of  the  body  and  become  somewhat  flattened 
at  their  free  ends,  remaining  cylindrical  in  their  proximal  portions. 
A  furrow  or  constriction  appears  at  the  junction  of  the  flattened  and 
cylindrical  portions  (Fig.  57),  and  later  a  second  constriction  divides 
the  cylindrical  portion  into  a  proximal  and  distal  moiety,  the  three 
segments  of  each  limb — the  arm,  forearm,  and  hand  in  the  upper 
limb,  and  the  thigh,  leg,  and  foot  in  the  lower — being  thus  marked 
out.  The  digits  are  first  indicated  by  the  development  of  four 
radiating  shallow  grooves  upon  the  hand  and  foot  regions  (Fig.  58), 


DEVELOPMENT    OF    THE    LIMBS  IOI 

and  a  transverse  furrow  uniting  the  proximal  ends  of  the  digital 
furrows  indicates  the  junction  of  the  digital  and  palmar  regions  of 
the  hand  or  of  the  toes  and  body  of  the  foot.  After  this  stage  is 
reached  the  development  of  the  upper  limb  proceeds  more  rapidly 
than  that  of  the  lower,  although  the  processes  are  essentially  the 
same  in  both  limbs.  The  digits  begin  to  project  slightly,  but  are  at 
first  to  a  very  considerable  extent  united  together  by  a  web,  whose 
further  growth,  however,  does  not  keep  pace  with  that  of  the  digits, 
these  thus  coming  to  project  more  and  more  in  later  stages.  Even 
in  comparatively  early  stages  the  thumb,  and  to  a  somewhat  slighter 
extent  the  great  toe,  is  widely  separated  from  the  second  digit 
(Figs.  59  and  60). 

While  these  changes  have  been  taking  place  the  entire  limbs 
have  altered  their  position  with  reference  to  the  axis  of  the  body, 
being  in  stages  later  than  that  shown  in  Fig.  57  directed  ventrally 
so  that  their  longitudinal  axes  are  at  right  angles  to  that  of  the  body. 
From  the  figures  of  later  stages  it  may  be  seen  that  it  is  the  thumb 
(radial)  side  of  the  arm  and  the  great  toe  (tibial)  side  of  the  leg 
which  are  directed  forward;  the  plantar  and  palmar  surfaces  of 
the  feet  and  hands  are  turned  toward  the  body  and  the  elbow  is 
directed  outward  and  slightly  backward,  while  the  knee  looks 
outward  and  slightly  forward.  It  seems  proper  to  conclude  that 
the  radial  side  of  the  arm  is  homologous  with  the  tibial  side  of  the 
leg,  the  palmar  surface  of  the  hand  with  the  plantar  surface  of  the 
foot,  and  the  elbow  with  the  knee. 

The  limbs  are,  however,  still  in  the  quadrupedal  condition,  and 
they  must  later  undergo  a  second  alteration  in  position  so  that  their 
long  axes  again  become  parallel  with  that  of  the  body.  This  is  accom- 
plished by  a  rotation  of  the  limbs  around  axes  passing  through  the 
shoulders  and  hip-joints,  together  with  a  rotation  about  their  longi- 
tudinal axes  through  an  angle  of  90  degrees.  This  axial  rotation  of 
the  upper  limb  is,  however,  in  exactly  the  opposite  direction  to  that 
of  the  lower  limb  of  the  corresponding  side,  so  that  the  homologous 
surfaces  of  the  two  limbs  have  entirely  different  relations,  the  radial 
side  of  the  arm,  for  instance,  being  the  outer  side  while  the  tibial  side 


102  AGE    OF    EMBRYO  AT   DIFFERENT    STAGES 

of  the  leg  is  the  inner  side,  and  whereas  the  palmar  surface  of  the 
hand  looks  ventrally,  the  plantar  surface  of  the  foot  looks  dorsally. 
In  making  these  statements  no  account  is  taken  of  the  secondary- 
position  which  the  hand  may  assume  as  the  result  of  its  pronation; 
the  positions  given  are  those  assumed  by  the  limbs  when  both  the 
bones  of  their  middle  segment  are  parallel  to  one  another. 

It  may  be  pointed  out  that  the  prevalent  use  of  the  physiological 
terms  flexor  and  extensor  to  describe  the  surfaces  of  the  limbs  has  a 
tendency  to  obscure  their  true  morphological  relationships.  Thus  if, 
as  is  usual,  the  dorsal  surface  of  the  arm  be  termed  its  extensor  surface, 
then  the  same  term  should  be  applied  to  the  entire  ventral  surface  of  the 
leg,  and  all  movements  of  the  lower  limb  ventrally  should  be  spoken  of  as 
movements  of  extension  and  any  movement  dorsally  as  movements  of 
flexion.  And  yet  a  ventral  movement  of  the  thigh  is  generally  spoken  of 
as  a  flexion  of  the  hip-joint,  while  a  straightening  out  of  the  foot  upon 
the  leg — that  is  to  say,  a  movement  of  it  dorsally — is  termed  its  extension. 

The  Age  of  the  Embryo  at  Different  Stages. — The  age  of  an 

embryo  must  be  dated  from  the  moment  of  fertilization  and  from 
what  has  been  said  in  preceding  pages  (pp.  27,  34)  it  is  evident  that 
it  must  be  difficult  to  determine  the  exact  date  of  this  event  from 
that  of  the  cessation  of  the  menses,  or  even  when  the  date  of  the 
coition  that  resulted  in  pregnancy  is  known.  And,  furthermore, 
not  only  is  the  actual  date  of  the  beginning  of  development  uncertain, 
but  in  the  majority  of  known  early  human  embryos  the  time  of  the 
cessation  of  development  is  also  more  or  less  uncertain,  since  so 
many  of  these  embryos  are  abortions  and  their  expulsion  need  not 
necessarily  have  immediately  succeeded  their  death. 

These  various  sources  of  uncertainty  are  of  especial  importance 
in  the  cases  of  embryos  in  the  early  stages  of  development,  when  a 
day  more  or  less  means  much,  and  it  seems  probable  that  many  of  the 
estimated  ages  given  for  young  embryos,  based  on  the  date  of  the 
last  menstruation,  are  too  low.  This  certainly  is  the  case  with  the 
ages  assigned  to  such  embryos  by  His,  who  estimated  embryos  of 
2.2  to  3.0  mm.  to  be  two  to  two  and  one-half  weeks  old,  those  of 
5.0  to  6.0  mm.  to  be  about  three  and  one-half  weeks  and  those  of 
10.0  to  11.0  mm.  to  be  about  four  and  one-half  weeks. 


AGE    OF    EMBRYO   AT   DIFFERENT    STAGES 


103 


There  are  on  record,  however,  a  few  cases  in  which  the  date  of  the 
fruitful  coition  is  definitely  known,  and  from  these,  few  though  they 
be,  somewhat  more  definite  information  may  be  obtained.  Thus 
it  is  fairly  certain  that  the  Bryce-Teacher  ovum,  with  an  embryo 
measuring  about  0.15  mm.  in  length,  was  the  result  of  a  coition 
which  took  place  sixteen  days  before  the  ovum  was  aborted,  and  one 
cannot  be  far  astray  in  assuming  the  embryo  to  be  about  two  weeks 
old.  Similarly,  an  embryo  described  by  Eternod  and  measuring 
1.3  mm.  in  length  was  the  result  of  a  single  coition  occurring  twenty- 
one  days  previously  and  its  age  may  be  set  at  approximately  three 
weeks  or  better  at  eighteen  or  nineteen  days.  A  later  embryo  in 
which  the  nape  bend  and  the  coiling  of  the  body  had  appeared  and 
which  measured  8.8  mm.  in  vertex-breech  length,  resulted  from  a 
single  coitus  that  took  place  thirty-eight  days  before  the  abortion, 
so  that  the  embryo  may  be  regarded  as  having  been  somewhat  more 
than  five  weeks  old.  These  and  two  other  similar  cases  may  be 
combined  into  a  table  thus: 


Length  of  embryo 

Days   intervening 

Probable   age   in 

Authority 

in  mm. 

between  coition 

days 

and  abortion 

About    0.15 

16J 

i3-J4 

Bryce-Teacher. 

i-3 

21 

18-19 

Eternod 

V.  B.     8.8 

38 

37 

Tandler. 

V.  B.  14.0 

47 

44-45 

Rabl. 

V.  B.  25.0 

56 

53-54 

Mall. 

If,  on  the  basis  of  these  figures,  one  may  venture  to  estimate  the 
age  of  embryos  of  other  lengths  those  of  2.0  to  3.0  mm.  may  be 
supposed  to  belong  to  the  fourth  week  of  development,  those  of 
5.0  to  6.0  vertex-breech  length  to  the  latter  part  of  the  fifth  week, 
those  of  10.0  mm.  to  the  end  of  the  sixth  week  and  those  of  25.0  to  28.0 
mm.  which  are  just  passing  into  the  fetus  stage,  to  the  end  of  the 
eighth  week.     As  regards  the  later  periods  of  development,  the 


104  LITERATURE 

limits  of  error  for  any  date  become  of  less  importance.     Schroder 
gives  the  following  measurements  as  the  average: 

3d  lunar  month 70-90  mm. 

4th  lunar  month ' 100-170  mm. 

5th  lunar  month 180-270  mm. 

6th  lunar  month 280-340  mm. 

7th  lunar  month 350-380  mm. 

8th  lunar  month 425  mm. 

9th  lunar  month 467  mm. 

10th  lunar  month 490-500  mm. 

The  data  concerning  the  weight  of  embryos  of  different  ages  are 
as  yet  very  insufficient,  and  it  is  well  known  that  the  weights  of  new- 
born children  may  vary  greatly,  the  authenticated  extremes  being, 
according  to  Vierordt,  717  grams  and  6123  grams.  It  is  probable 
that  considerable  variations  in  weight  occur  also  during  fetal  life. 
So  far  as  embryos  of  the  first  two  months  are  concerned,  the  data  are 
too  imperfect  for  tabulation;  for  later  periods  Fehling  gives  the 
following  as  average  weights: 

3d   month 20  grams. 

4th  month 120  grams. 

5th  month 285  grams. 

6th  month 635  grams. 

7th  month 1220  grams. 

8th  month 1700  grams. 

9th  month 2240  grams. 

10th  month 325°  grams. 

and  the  results  obtained  by  Jackson  are  essentially  similar. 

LITERATURE. 

In  addition  to  the  papers  of  Bryce  and  Teacher,  Eternod,  Fetzer,  Frassi,  Herzog, 
Peters,  Von  Spee  and  Strahl  and  Beneke  cited  in  the  preceding  chapter,  the  following 
may  be  mentioned: 

Bremer:  "Description  of  a  4  mm.  Human  Embryo,"  Amer.  Journ.  Anal.,  v,  1906. 
J.  Broman:  "Beobachtung  eines  menschlichen  Embryos  von  beinahe  3  mm.  Lange 

mit   specieller   Bemerkung  uber   die   bei   demselben   befindlichen   Hirnfalten," 

Morpholog.  Arbeiten,  v,  1895. 
A.  J.  P.  van  den  Broek:  "Zur  Kasuistik  junger  menschlicher  Embryonen,"  Anal,. 

Hefte,  xliv,  191 1. 


LITERATURE  105 

J.  M.  Coste:  "  Histoire  generale  et  particuliere  du  developpement  des  corps  organises," 

Paris,  1847-1859. 
W.  E.  Dandy:  "A  Human  Embryo  with  Seven  Pairs  of  Somites,  Measuring  about 

2  mm.  in  Length,"  Amer.  Joiirn.  Anal.,  x,  1910. 
A.   Ecker:  "Beitrage  zur  Kenntniss   der  ausserer  Formen  jiingster  menschlichen 

Embryonen,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  18S0. 
C.  Elze:  "  Beschreibung  eines  menschlichen  Embryos  von  zirka  7  mm.  grosster  Lange," 

Anat.  Hefte,  xxxv,  1907. 
C.  Giacomini:  "Un  ceuf  humain  de  11  jours,"  Archives  Hal.  de  Biologie,  xxix,  1898. 
V.    Hensen:    "Beitrag    zur   Morphologie    der    Korperform   und   des    Gehirns    des 

menschlichen  Embryos,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1877. 
W.  His:  "Anatomie  menschlicher  Embryonen,"  Leipzig,  1880. 
F.  Hochstetter:  "Bilder  der  ausseren  Korperform  einiger  menschlicher  Embryonen 

aus  den  beiden  Ersten  Monaten  der  Entwicklung,"  Munich,  1907. 
N.  W.  Ingalls:  "Beschreibung  eines  menschlichen  Embryos  von  4.9  mm.,"  Arch. 

fiir  mikr.  Anat.,  lxx,  1907. 
C.  M.  Jackson:  "  On  the  Prenatal  Growth  of  the  Human  Body  and  the  Relative  Growth 

of  the  Various  Organs  and  Parts,"  Amer.  Journ.  Anat.,  ix,  1909. 
J.  Janosik:  "Zwei  junge  menschliche  Embryonen,"  Archiv  fiir  mikrosk.  Anat.,  xxx, 

1887. 
H.  E    Jordan:  "Description  of  a  5  mm.  Human  Embryo,"  Anat.  Record,  ill,  1909. 
P.  Jung:  "Beitrage  zur  friihesten  Ei-einbettung  beim  menschlichen  Weibe,"  Berlin, 

1908. 
F.  Keibel:  "Ein  sehr  junges  menschliches  Ei,"  Archiv  fiir  Anat.  und  Physiol.,  Anat. 

Abth.,  1890. 
F.   Keibel:  "Ueber  einen   menschlichen   Embryo   von   6.8   mm.   grosster  Lange," 

Verhandl.  Anatom.  Gesellsch.,  xiii,  1899. 
F.  Keibel  and  C.  Elze:  "  Normentafeln  zur  Entwicklungsgeschichte  der  Wirbeltiere," 

Heft  viii,  1 90S. 
J.   Kollmann:  "Die  Korperform  menschlicher  normaler  und  pathologischer  Em- 
bryonen," Archiv  fur  Anat.  und  Physiol.,  Anat  Abth.,  Supplement,  18S9. 
A.  Low:  "Description  of  a  Human  Embryo  of  13-14  Mesodermic  Somites,"  Journ. 

Anat.  and  Phys.,  xlii,  1908. 
F.  P.  Mall:  "A  Human  Embryo  Twenty-six  Days  Old,"  Journ.  of  Morphology,  V, 

1891. 
F.  P.  Mall:  "A  Human  Embryo  of  the  Second  Week,"  Anat.  Anzeiger,  viii,  1893. 
F.  P.  Mall:  "Early  Human  Embryos  and  the  Mode  of  their  Preservation,"  Bulletin  of 

the  Johns  Hopkins  Hospital,  XV,  1S94. 
C.  S.  Minot:  "Human  Embryology,"  New  York,  1892. 
J.  Muller:  " Zergliederungen  menschlicher  Embryonen  aus  friiherer  Zeit,"  Archiv 

fiir  Anat.  und  Physiol.,  1830. 
C.  Phisalix:  "Etude  d'un  Embryon  humain  de  11  millimeters,"  Archives  de  zoolog. 

experimentale  et  generale,  Ser.  2,  vi,  1888. 
H.  Piper:  "Ein  menschlicher  Embryo  von  6.8  mm.  Nackenlinie,"  Archiv  fiir  Anat. 

und  Physiol.,  Anat.  Abth,,  1898. 


106  LITERATURE 

C.   Rabl:  "Die  Entwicklung  des   Gesichtes,   Heft  i,   Das  Gesicht  der  Saugetiere, 

Leipzig,  1902. 
G.    Retzitts:  "Zur   Kenntniss   der   Entwicklung    der  Korperformen  des  Menschen 

wahrend  der  fotalen  Lebensstufen,"  Biolog.  Untersuch.,  xi,  1904. 
J.  Tandler:  "Ueber  einen  menschlichen  Embryo  von  38  Tage,"  Anat.  Anzeiger, 

xxxi,  1907. 
Allen  Thompson:  "Contributions  to  the  History  of  the  Structure  of  the  Human 

Ovum  and  Embryo  before  the  Third  Week  after  Conception,  with  a  Description 

of  Some  Early  Ova,"  Edinburgh  Med.  and  Surg.  Journal,  in,  1839.     (See  also 

Froriep's  Neue  Notizen,  xiu,  1840.) 
P.  Thompson:  "Description  of  a  human  embryo  of  twenty-three  paired  somites," 

Journ.  Anat.  and  Phys.,  xli,  1907. 


CHAPTER  V. 

THE  YOLK -STALK,   BELLY-STALK,  AND  FETAL 
MEMBRANES. 

The  conditions  to  which  the  embryos  and  larvse  of  the  majority 
of  animals  must  adapt  themselves  are  so  different  from  those  under 
which  the  adult  organisms  exist  that  in  the  early  stages  of  develop- 
ment special  organs  are  very  frequently  developed  which  are  of  use 
only  during  the  embryonic  or  larval  period  and  are  discarded  when 
more  advanced  stages  of  development  have  been  reached.  This 
remark  applies  with  especial  force  to  the  human  embryo  which  leads 
for  a  period  of  nine  months  what  may  be  termed  a  parasitic  existence, 
drawing  its  nutrition  from  and  yielding  up  its  waste  products  to  the 
blood  of  the  parent.  In-  order  that  this  may  be  accomplished  cer- 
tain special  organs  are  developed  by  the  embryo,  by  means  of  which 
it  forms  an  intimate  connection  with  the  walls  of  the  uterus,  which, 
on  its  part,  becomes  greatly  modified,  the  combination  of  embryonic 
and  maternal  structures  producing  what  are  termed  the  deciduce, 
owing  to  their  being  discarded  at  birth  when  the  parasitic  mode  of 
life  is  given  up. 

Furthermore,  it  has  already  been  seen  that  many  peculiar  modi- 
fications of  development  in  the  human  embryo  result  from  the  inheri- 
tance of  structures  from  more  or  less  remote  ancestors,  and  among 
the  embryonic  adnexes  are  found  structures  which  represent  in  a 
more  or  less  modified  condition  organs  of  considerable  functional 
importance  in  lower  forms.  Such  structures  are  the  yolk-stalk  and 
vesicle,  the  amnion,  and  the  allantois,  and  for  their  proper  under- 
standing it  will  be  well  to  consider  briefly  their  development  in  some 
lower  form,  such  as  the  chick. 

At  the  time  when  the  embryo  of  the  chick  begins  to  be  con- 
stricted off  from  the  surface  of  the  large  yolk-mass,  a  fold,  consisting 

107 


io8 


YOLK-STALK   AND    FETAL    MEMBRANES 


of  ectoderm  and  somatic  mesoderm,  arises  just  outside  the  embryonic 
area,  which  it  completely  surrounds.  As  development  proceeds  the 
fold  becomes  higher  and  its  edges  gradually  draw  nearer  together 
over  the  dorsal  surface  of  the  embryo  (Fig.  64,  A,  Af),  and  finally 
meet  and  fuse  (Fig.  64,  B  and  C),  so  that  the  embryo  becomes 
enclosed  within  a  sac,  which  is  termed  the  amnion  and  is  formed  by 
the  fusion  of  the  layers  which  constituted  the  inner  wall  of  the  fold. 
The  layers  of  the  outer  wall  of  the  fold  after  fusion  form  part  of  the 


Fig.  64. — Diagrams  Illustrating  the  Formation  of  the  Amnion  and  Allantois 

in  the  Chick. 
Af,  Amnion  folds;  Al,  allantois;  Am,  amniotic  cavity;  Ds,  yolk-sac. — (Cegenbaur.) 


general  ectoderm  and  somatic  mesoderm  which  make  up  the  outer 
wall  of  the  ovum  and  together  are  known  as  the  serosa,  correspond- 
ing to  the  chorion  of  the  mammalian  embryo.  The  space  which 
occurs  between  the  amnion  and  the  serosa  is  a  portion  of  the  extra- 
embryonic ccelom  and  is  continuous  with  the  embryonic  pleuro- 
peritoneal  cavity. 

In  the  ovum  of  the  chick,  as  in  that  of  the  reptile,  the  proto- 
plasmic material  is  limited  to  one  pole  and  rests  upon  the  large  yolk- 


THE  AMNION  IO9 

mass.  As  development  proceeds  the  germ  layers  gradually  extend 
around  the  yolk-mass  and  eventually  completely  enclose  it,  the  yolk- 
mass  coming  to  lie  within  the  endodermal  layer,  which,  together 
with  the  splanchnic  mesoderm  which  lines  it,  forms  what  is  termed 
the  yolk-sac.  As  the  embryo  separates  from  the  yolk-mass  the  yolk- 
sac  is  constricted  in  its  proximal  portion  and  so  differentiated  into  a 
yolk-stalk  and  a  yolk-sac,  the  contents  of  the  latter  being  gradually 
absorbed  by  the  embryo  during  its  growth,  its  walls  and  those  of  the 
stalk  being  converted  into  a  portion  of  the  embryonic  digestive  tract. 

In  the  meantime,  however,  from  the  posterior  portion  of  the 
digestive  tract,  behind  the  point  of  attachment  of  the  yolk-sac,  a 
diverticulum  has  begun  to  form  (Fig.  64,  A,  Al).  This  increases  in 
size,  projecting  into  the  extra-embryonic  portion  of  the  pleuroperi- 
toneal  cavity  and  pushing  before  it  the  splanchnic  mesoderm  which 
lines  the  endoderm  (Fig.  64,  B  and  C) .  This  is  the  allantois,  which, 
reaching  a  very  considerable  size  in  the  chick  and  applying  itself 
closely  to  the  inside  of  the  serosa,  serves  as  a  respiratory  and  excre- 
tory organ  for  the  embryo,  for  which  purpose  its  walls  are  richly 
supplied  with  blood-vessels,  the  allantoic  arteries  and  veins. 

Toward  the  end  of  the  incubation  period  both  the  amnion  and 
allantois  begin  to  undergo  retrogressive  changes,  and  just  before 
the  hatching  of  the  young  chick  they  become  completely  dried  up 
and  closely  adherent  to  the  egg-shell,  at  the  same  time  separating 
from  their  point  of  attachment  to  the  body  of  the  young  chick,  so 
that  when  the  chick  leaves  the  egg-shell  it  bursts  through  the  dried- 
up  membranes  and  leaves  them  behind  as  useless  structures. 

The  Amnion. — Turning  now  to  the  human  embryo,  it  will  be 
found  that  the  same  organs  are  present,  though  somewhat  modified 
either  in  the  mode  or  the  extent  of  their  development.  A  well- 
developed  amnion  occurs,  arising,  however,  in  a  very  different  man- 
ner from  what  it  does  in  the  chick;  a  large  yolk-sac  occurs  even 
though  it  contains  no  yolk;  and  an  allantois  which  has  no  respiratory 
or  excretory  functions  is  present,  though  in  a  somewhat  degenerated 
condition.  It  has  been  seen  from  the  description  of  the  earliest 
stages  of  development  that  the  processes  which  occur  in  the  lowe 


IIO  THE  AMNION 

forms  are  greatly  abbreviated  in  the  human  embryo.  The  envelop- 
ing layer,  instead  of  gradually  extending  from  one  pole  to  enclose 
the  entire  ovum,  develops  in  situ  during  the  stages  immediately 
succeeding  segmentation,  and  the  extra-embryonic  mesoderm, 
instead  of  growing  out  from  the  embryo  to  enclose  the  yolk-sac, 
splits  off  directly  from  the  enveloping  layer.  The  earliest  stages  in 
the  development  of  the  amnion  are  not  yet  known  for  the  human 
embryo,  but  from  the  condition  in  which  it  is  found  in  the  Peters 
embryo  (Fig.  37)  and  in  the  embryo  v.H.  of  von  Spee  (Fig.  39)  it 
is  probable  that  it  arises,  not  by  the  fusion  of  the  edges  of  a  fold,  as 
in  the  chick,  but  by  a  vacuolization  of  a  portion  of  the  inner  cell- 
mass,  as  has  been  described  as  occurring  in  the  bat  (p.  54).  It  is, 
then,  a  closed  cavity  from  the  very  beginning,  the  floor  of  the  cavity 
being  formed  by  the  embryonic  disk,  its  posterior  wall  by  the 
anterior  surface  of  the  belly-stalk,  while  its  roof  and  sides  are  thin 
and  composed  of  a  single  layer  of  flattened  ectodermal  cells  lined 
on  the  outside  by  a  layer  of  mesoderm  continuous  with  the  somatic 
mesoderm  of  the  embryo  and  the  mesoderm  of  the  belly-stalk 
(Fig.  65,  A). 

When  the  bending  downward  of  the  peripheral  portions  of  the 
embryonic  disk  to  close  in  the  ventral  surface  of  the  embryo  occurs, 
the  line  of  attachment  of  the  amnion  to  the  disk  is  also  carried 
ventrally  (Fig.  65,  B),  so  that  when  the  constriction  off  of  the  embryo 
is  practically  completed,  the  amnion  is  attached  anteriorly  to  the 
margin  of  the  umbilicus  and  posteriorly  to  the  extremity  of  the  band 
of  ectoderm  lining  what  may  now  be  considered  the  posterior 
surface  of  the  belly-stalk,  while  at  the  sides  it  is  attached  along  an 
oblique  line  joining  these  two  points  (Fig.  65,  B  and  C,  in  which  the 
attachment  of  the  amnion  is  indicated  by  the  broken  line). 

Leaving  aside  for  the  present  the  changes  which  occur  in  the 
attachment  of  the  amnion  to  the  embryo  (see  p.  116),  it  may  be 
said  that  during  the  later  growth  of  the  embryo  the  amniotic  cavity 
increases  in  size  until  finally  its  wall  comes  into  contact  with  the 
chorion,  the  extra-embryonic  body-cavity  being  thus  practically 
obliterated  (Fig.  65,  D),  though  no  actual  fusion  of  amnion  and 


THE  AMNION 


III 


chorion  occurs.  Suspended  by  the  umbilical  cord,  which  has  by 
this  time  developed,  the  embryo  floats  freely  in  the  amniotic  cavity, 
which  is  filled  by  a  fluid,  the  liquor  amnii,  whose  origin  is  involved 
in  doubt,  some  authors  maintaining  that  it  infiltrates  into  the  cavity 
from  the  maternal  tissues,  while  others  hold  that  a  certain  amount 


Fig.  65. — Diagrams  Illustrating  the  Formation  of  the  Umbilical  Cord. 

The  heavy  black  line  represents  the  embryonic  ectoderm;  the  dotted  line  represents 
the  line  of  reflexion  of  the  body  ectoderm  into  that  of  the  amnion.  Ac,  Amniotic  cavity ; 
Al,  allantois;  Be,  extra-embryonic  ccelom;  Bs,  belly-stalk;  Ch,  chorion;  P,  placenta;  Uc, 
umbilical  cord;  V,  chorionic  villi;  Ys,  yolk-sac. 


of  it  at  least  is  derived  from  the  embryo.  It  is  a  fluid  with  a  specific 
gravity  of  about  1.003  an(^  contains  about  1  per  cent,  of  solids, 
principally  albumin,  grape-sugar,  and  urea,  the  last  constituent 
probably  coming  from  the  embryo.  When  present  in  greatest 
quantity — that  is  to  say,  at  about  the  beginning  of  the  last  month 


112  THE    YOLK-SAC 

of  pregnancy — it  varies  in  amount  between  one-half  and  three- 
fourths  of  a  liter,  but  during  the  last  month  it  diminishes  to  about 
half  that  quantity.  To  protect  the  epidermis  of  the  fetus  from 
maceration  during  its  prolonged  immersion  in  the  liquor  amnii,  the 
sebaceous  glands  of  the  skin  at  about  the  sixth  month  of  develop- 
ment pour  out  upon  the  surface  of  the  body  a  white  fatty  secretion 
known  as  the  vernix  caseosa. 

During  parturition  the  amnion,  as  a  rule,  ruptures  as  the  result 
of  the  contraction  of  the  uterine  walls  and  the  liquor  amnii  escapes 
as  the  "waters,"  a  phenomenon  which  normally  precedes  the 
delivery  of  the  child.  As  a  rule,  the  rupture  is  sufficiently  extensive 
to  allow  the  passage  of  the  child,  the  amnion  remaining  behind  in 
the  uterus,  to  be  subsequently  expelled  along  with  the  deciduae. 

Occasionally  it  happens,  however,  that  the  amnion  is  sufficiently 
strong  to  withstand  the  pressure  exerted  upon  it  by  the  uterine  contractions 
and  the  child  is  born  still  enveloped  in  the  amnion,  which,  in  such  cases, 
is  popularly  known  as  the  "caul,"  the  possession  of  which,  according  to 
an  old  superstition,  marks  the  child  as  a  favorite  of  fortune. 

As  stated  above,  the  liquor  amnii  varies  considerably  in  amount  in 
different  cases,  and  occasionally  it  may  be  present  in  excessive  quantities, 
producing  a  condition  known  as  hydramnios.  On  the  other  hand,  the 
amount  may  fall  considerably  below  the  normal,  in  which  case  the  amnion 
may  form  abnormal  unions  with  the  embryo,  sometimes  producing 
malformations.  Occasionally  also  bands  of  a  fibrous  character  traverse 
the  amniotic  cavity  and,  tightening  upon  the  embryo  during  its  growth, 
may  produce  various  malformations,  such  as  scars,  splitting  of  the  eyelids 
or  lips,  or  even  amputation  of  a  limb. 

The  Yolk-sac. — The  probable  mode  of  development  of  the 
yolk-sac  in  the  human  embryo,  and  its  differentiation  into  yolk-stalk 
and  yolk- vesicle  have  already  been  described  (p.  86).  When  these 
changes  have  been  completed,  the  vesicle  is  a  small  pyriform  structure 
lying  between  the  amnion  and  the  chorionic  mesoderm,  some  dis- 
tance away  from  the  extremity  of  the  umbilical  cord  (Fig.  65,  D), 
and  the  stalk  is  a  long  slender  column  of  cells  extending  from  the 
vesicle  through  the  umbilical  cord  to  unite  with  the  intestinal 
tract  of  the  embryo.  The  vesicle  persists  until  birth  and  may  be 
found  among  the  decidual  tissues  as  a  small  sac  measuring  from  3  to 


THE  ALLANTOIS   AND    BELLY-STALK  II3 

10  mm.  in  its  longest  diameter.  The  stalk,  however,  early  under- 
goes degeneration,  the  lumen  which  it  at  first  contains  becoming 
obliterated  and  its  endoderm  also  disappearing  as  early  as  the  end 
of  the  second  month  of  development.  The  portion  of  the  stalk 
which  extends  from  the  umbilicus  to  the  intestine  usually  shares  in 
the  degeneration  and  disappears,  but  in  about  3  per  cent,  of  cases  it 
persists,  forming  a  more  or  less  extensive  diverticulum  of  the  lower 
part  of  the  small  intestine,  sometimes  only  half  an  inch  or  so  in 
length  and  sometimes  much  larger.  It  may  or  may  not  retain  con- 
nection with  the  abdominal  wall  at  the  umbilicus,  and  is  known  as 
Meckel's  diverticulum. 

This  embryonic  rudiment  is  of  no  little  importance,  since,  when 
present,  it  is  apt  to  undergo  invagination  into  the  lumen  of  the  small 
intestine  and  so  occlude  it.  How  frequently  this  happens  relatively  to 
the  occurrence  of  the  diverticulum  may  be  judged  from  the  fact  that  out 
of  one  hundred  cases  of  occlusion  of  the  small  intestine  six  were  due  to  an 
invagination  of  the  diverticulum. 

In  the  reptiles  and  birds  the  yolk-sac  is  abundantly  supplied  with 
blood-vessels  by  means  of  which  the  absorption  of  the  yolk  is  carried 
on,  and  even  although  the  functional  importance  of  the  yolk-sac  as 
an  organ  of  nutrition  is  almost  nil  in  the  human  embryo,  yet  it 
still  retains  a  well-developed  blood-supply,  the  walls  of  the  vesicle, 
especially  possessing  a  rich  network  of  vessels.  The  future  history 
of  these  vessels,  which  are  known  as  the  vitelline  vessels,  will  be 
described  later  on. 

The  Allantois  and  Belly-stalk. — It  has  been  seen  that  in 
reptilian  and  avian  embryos  the  allantois  reaches  a  high  degree  of 
development  and  functions  as  a  respiratory  and  excretory  organ  by 
coming  into  contact  with  what  is  comparable  to  the  chorion  of  the 
mammalian  embryo.  In  man  it  is  very  much  modified  both  in  its 
mode  of  development  and  in  its  relations  to  other  parts,  so  that  its 
resemblance  to  the  avian  organ  is  somewhat  obscured.  The  differ- 
ences depend  partly  upon  the  remarkable  abbreviation  manifested 
in  the  early  development  of  the  human  embryo  and  partly  upon  the 
fact  that  the  allantois  serves  to  place  the  embryo  in  relation  with  the 
8 


U4 


THE .  ALLANTOIS   AND    BELLY-STALK 


maternal  blood,  instead  of  with  the  external  atmosphere,  as  is  the 
case  in  the  egg-laying  forms.  Thus,  the  endodermal  portion  of  the 
allantois,  instead  of  arising  from  the  intestine  and  pushing  before 
it  a  layer  of  splanchnic  mesoderm  to  form  a  large  sac  lying  freely  in 
the  extra-embryonic  portion  of  the  body-cavity,  appears  in  the  human 
embryo  before  the  intestine  has  differentiated  from  the  yolk-sac  and 
pushes  its  way  into  the  solid  mass  of  mesoderm  which  forms  the 
belly-stalk  (Fig.  65,  A).  To  understand  the  significance  of  this  proc- 
ess it  is  necessary  to  recall  the  abbreviation  in  the  human  embryo  of 
the  development  of  the  extra-embryonic  mesoderm  and  body-cavity. 
Instead  of  growing  out  from  the  embryonic  area,  as  it  does  in  the 
lower  forms,  this  mesoderm  develops  in  situ  by  splitting  off  from 
the  layer  of  enveloping  cells  and,  furthermore,  the  extra-embryonic 

body-cavity  arises  by  a  splitting  of  the 
mesoderm  so  formed  before  there  is  any 
trace  of  a  splitting  of  the  embryonic 
mesoderm  (Fig.  38).  The  belly-stalk, 
whose  development  from  a  portion  of 
the  inner  cell-mass  has  already  been 
traced  (p.  68),  is  to  be  regarded  as  a 
portion  of  the  body  of  the  embryo, 
since  the  ectoderm  which  covers  one 
surface  of  it  resembles  exactly  that  of 
the  embryonic  disk  and  shows  an  ex- 
tension backward  of  the  medullary 
groove  upon  its  surface  (Fig.  66).  The 
mesoderm,  therefore,  of  the  belly-stalk 
is  to  be  regarded  as  a  portion  of  the  embryonic  mesoderm  which  has 
not  yet  undergone  a  splitting  into  somatic  and  splanchnic  layers, 
and,  indeed,  it  never  does  undergo  such  a  splitting,  so  that  there  is 
no  body-cavity  into  which  the  endodermal  allantoic  diverticulum 
can  grow. 

But  this  does  not  account  for  all  the  peculiarities  of  the  human 
allantois.  In  the  birds,  and  indeed  in  the  lower  oviparous  mammals, 
the  endodermal  portion  of  the  allantois  is  equally  developed  with 


Fig.  66. — Transverse  Sec- 
tion THROUGH  THE  BELLY-STALK 

of  an  Embryo  of  2.15  mm. 

Aa,  Umbilical  (allantoic) 
artery;  All,  allantois;  am,  am- 
nion; Va,  umbilical  (allantoic) 
vein. — (His.) 


THE   ALLANTOIS   AND    BELLY-STALK  115 

the  mesodermal  portion,  the  allantois  being  an  extensive  sac  whose 
cavity  is  rilled  with  fluid,  and  this  is  also  true  of  such  mammals  as 
the  marsupials,  the  rabbit,  and  the  ruminants.  In  man,  however, 
the  endodermal  diverticulum  never  becomes  a  sac-like  structure, 
but  is  a  slender  tube  extending  from  the  intestine  to  the  chorion  and 
lying  in  the  substance  of  the  mesoderm  of  the  belly-stalk  (Fig.  65, 
D),  the  greater  portion  of  which  is  to  be  regarded  as  homologous 
with  the  relatively  thin  layer  of  splanchnic  mesoderm  covering  the 
endodermal  diverticulum  of  the  chick.  An  explanation  of  this 
disparity  in  the  development  of  the  mesodermal  and  endodermal 
portions  of  the  human  allantois  is  perhaps  to  be  found  in  the  altered 
conditions  under  which  the  respiration  and  secretion  take  place. 
In  all  forms,  the  lower  as  well  as  the  higher,  it  is  the  mesoderm  which 
is  the  more  important  constituent  of  the  allantois,  since  in  it  the 
blood-vessels,  upon  whose  presence  the  physiological  functions 
depend,  arise  and  are  embedded.  In  the  birds  and  oviparous 
mammals  there  are  no  means  by  which  excreted  material  can  be 
passed  to  the  exterior  of  the  ovum,  and  it  is,  therefore,  stored  up 
within  the  cavity  of  the  allantois,  the  allantoic  fluid  containing 
considerable  quantities  of  nitrogen,  indicating  the  presence  of  urea. 
In  the  higher  mammals  the  intimate  relations  which  develop  between 
the  chorion  and  the  uterine  walls  allow  of  the  passage  of  excreted 
fluids  into  the  maternal  blood;  and  the  more  intimate  these  relations, 
the  less  necessity  there  is  for  an  allantoic  cavity  in  which  excreted 
fluid  may  be  stored  up.  The  difference  in  the  development  of  the 
cavity  in  the  ruminants,  for  example,  and  man  depends  probably 
upon  the  greater  intimacy  of  the  union  between  ovum  and  uterus 
in  the  latter,  the  arrangement  for  the  passage  of  the  excreted  material 
into  the  maternal  blood  being  so  perfect  that  there  is  practically  no 
need  for  the  development  of  an  allantoic  cavity. 

The  portion  of  the  endodermal  diverticulum  which  is  enclosed 
within  the  umbilical  cord  persists  until  birth  in  a  more  or  less 
rudimentary  condition,  but  the  intra-embryonic  portion  extending 
from  the  apex  of  the  bladder  to  the  umbilicus  becomes  converted 
into  a  solid  cord  of  fibrous  tissue  termed  the  urachus. 


Il6  THE    UMBILICAL   CORD 

Occasionally  a  lumen  persists  in  the  urachal  portion  of  the  allantois 
and  may  open  to  the  exterior  at  the  umbilicus,  in  which  case  urine  from 
the  bladder  may  escape  at  the  umbilicus. 

Since  the  allantois  in  the  human  embryo,  as  well  as  in  the  lower 
forms,  is  responsible  for  respiration  and  excretion,  its  blood-vessels 
are  well  developed.  They  are  represented  in  the  belly-stalk  by 
two  veins  and  two  arteries  (Fig.  66),  known  in  human  embryology 
as  the  umbilical  veins  and  arteries.  These  extend  from  the  body  of 
the  embryo  out  to  the  chorion,  there  branching  repeatedly  to  enter 
the  numerous  chorionic  villi  by  which  the  embryonic  tissues  are 
placed  in  relation  with  the  maternal. 

The  Umbilical  Cord. — During  the  process  of  closing  in  of  the 
ventral  surface  of  the  embryo  a  stage  is  reached  in  which  the  em- 
bryonic and  extra-embryonic  portions  of  the  body-cavity  are 
completely  separated  except  for  a  small  area,  the  umbilicus,  through 
which  the  yolk-stalk  passes  out  (Fig.  65,  B).  At  the  edges  of  this 
area  in  front  and  at  the  sides  the  embryonic  ectoderm  and  somatic 
mesoderm  become  continuous  with  the  corresponding  layers  of  the 
amnion,  but  posteriorly  the  line  of  attachment  of  the  amnion  passes 
up  upon  the  sides  of  the  belly-stalk  (Fig.  65,  B),  so  that  the  whole  of 
the  ventral  surface  of  the  stalk  is  entirely  uncovered  by  ectoderm, 
this  layer  being  limited  to  its  dorsal  surface  (Fig.  66).  In  sub- 
sequent stages  the  embryonic  ectoderm  and  somatic  mesoderm  at 
the  edges  of  the  umbilicus  grow  out  ventrally,  carrying  with  them 
the  line  of  attachment  of  the  amnion  and  forming  a  tube  which 
encloses  the  proximal  part  of  the  yolk-stalk.  The  ectoderm  of  the 
belly-stalk  at  the  same  time  extending  more  laterally,  the  condition 
represented  in  Fig.  65,  C,  is  produced,  and,  these  processes  con- 
tinuing, the  entire  belly-stalk,  together  with  the  yolk-stalk,  becomes 
enclosed  within  a  cylindrical  cord  extending  from  the  ventral 
surface  of  the  body  to  the  chorion  and  forming  the  umbilical  cord 
(Fig.  65,  D). 

From  this  mode  of  development  it  is  evident  that  the  cord  is, 
strictly  speaking,  a  portion  of  the  embryo,  its  surfaces  being  com- 
pletely covered  by  embryonic  ectoderm,  the  amnion  being  carried 


THE    UMBILICAL    CORD 


117 


-uv 


ua. 


lev 


Fig.  67. — -Transverse  Sections  of  the  Umbilical  Cord  of  Embryos  of  (A)  1.8  cm. 

and  (B)  25  cm. 
al,  Allantois;  c,  coelom;  ua,  umbilical  artery;  uv,  umbilical  vein;  ys,  yolk-stalk. 


Il8  THE   CHORION 

during  its  formation  further  and  further  from  the  umbilicus  until 
finally  it  is  attached  around  the  distal  extremity  of  the  cord. 

In  enclosing  the  yolk-stalk  the  umbilical  cord  encloses  also  a 
small  portion  of  what  was  originally  the  extra-embryonic  body- 
cavity  surrounding  the  yolk-stalk.  A  section  of  the  cord  in  an  early 
stage  of  its  development  (Fig.  67,  A)  will  show  a  thick  mass  of 
mesoderm  occupying  its  dorsal  region;  this  represents  the  mesoderm 
of  the  belly-stalk  and  contains  the  allantois  and  the  umbilical 
arteries  and  vein  (the  two  veins  originally  present  in  the  belly-stalk 
having  fused),  while  toward  the  ventral  surface  there  will  be  seen  a 
distinct  cavity  in  which  lies  the  yolk-stalk  with  its  accompanying 
blood-vessels.  The  portion  of  this  ccelom  nearest  the  body  of  the 
embryo  becomes  much  enlarged,  and  during  the  second  month  of 
development  contains  some  coils  of  the  small  intestine,  but  later  the 
entire  cavity  becomes  more  and  more  encroached  upon  by  the 
growth  of  the  mesoderm,  and  at  about  the  fourth  month  is  entirely 
obliterated.  A  section  of  the  cord  subsequent  to  that  period  of 
development  will  show  a  solid  mass  of  mesoderm  in  which  are 
embedded  the  umbilical  arteries  and  vein,  the  allantois,  and  the 
rudiments  of  the  yolk-stalk  (Fig.  67,  B). 

When  fully  formed,  the  umbilical  cord  measures  on  the  average 
55  cm.  in  length,  though  it  varies  considerably  in  different  cases,  and 
has  a  diameter  of  about  1.5  cm.  It  presents  the  appearance  of  being 
spirally  twisted,  an  appearance  largely  due,  however,  to  the  spiral 
course  pursued  by  the  umbilical  arteries,  though  the  entire  cord  may 
undergo  a  certain  amount  of  torsion  from  the  movements  of  the 
embryo  in  the  later  stages  of  development  and  may  even  be  knotted. 
The  greater  part  of  its  substance  is  formed  by  the  mesoderm,  the 
cells  of  which  become  stellate  and  form  a  recticulum,  the  meshes 
of  which  are  occupied  by  connective-tissue  fibrils  and  a  mucous  fluid 
which  gives  to  the  tissue  a  jelly-like  consistence,  whence  it  has  re- 
ceived the  name  of  Wharton's  jelly. 

The  Chorion. — To  understand  the  developmental  changes 
which  the  chorion  undergoes  it  will  be  of  advantage  to  obtain  some 
insight  into  the  manner  in  which  the  ovum  becomes  implanted  in 


THE    CHORION 


II9 


the  wall  of  the  uterus.  Nothing  is  known  as  to  how  this  implanta- 
tion is  effected  in  the  case  of  the  human  ovum;  it  has  already  been 
accomplished  in  the  youngest  ovum  at  present  known.  But  the 
process  has  been  observed  in  other  mammals,  and  what  takes  place 
in  Spermophilus,  for  example,  may  be  supposed  to  give  a  clue  to 
what  occurs  in  the  human  ovum.  In  the  spermophile  the  ovum 
lies  free  in  the  uterine  cavity  up  to  a  stage  at  which  the  vacuolization 


* I 


Fig.  68. — Successive  Stages  in  the  Implantation  of  the  Ovum  of  the  Spermophile  . 
a,  syncytial  knob;  k,  inner  cell-mass. — (Rejsek.) 

of  the  central  cells  is  almost  completed  (Fig.  68,  A).  At  one  region 
of  the  covering  layer  the  cells  become  thicker  and  later  form  a  syn- 
cytial projection  or  knob  which  comes  into  contact  with  the  uterine 
mucosa  (Fig.  68,  B),  and  at  the  point  of  contact  the  mucosa  cells 
undergo  degeneration,  allowing  the  knob  to  come  into  relation  with 
the  deeper  tissues  of  the  uterus  (Fig.  68,  C),  the  process  apparently 
being  one  in  which  the  mucosa  cells  are  eroded  by  the  syncytial  knob. 
It  seems  probable  that  in  the  human  ovum  the  process  is  at  first 
of  a  similar  nature  and  that  as  the  covering  layer  cells  come  into 


120 


THE   CHORION 


c 

Fig.  69. — Diagrams  Illustrating  the  Implantation  of  the  Ovum. 
ac,  amniotic  cavity;  bs,  belly-stalk;  cf,  chorion  frondosum;  cl,  chorion  laeve;Jc, 
decidua  capsularis;  ic,  inner  cell-mass;  s,  space  surrounding  ovum  which  becomes  the 
intervillous  space;  um,  uterine  mucosa;  v,  chorionic  villus;  ys,  yolk-sac. 


THE    CHORION  121 

contact  with  the  deeper  layers  of  the  uterus,  these  too  are  eroded,  and, 
the  uterine  blood-vessels  being  included  in  the  erosion  process,  an 
extravasation  of  blood  plasma  and  corpuscles  occurs  in  the  vicinity 
of  the  burrowing  ovum.  In  the  meantime  the  ovum  has  increased 
considerably  in  size,  its  growth  in  these  early  stages  being  especially 
rapid,  and  the  area  of  contact  consequently  increases  in  size,  entailing 
continued  erosion  of  the  uterine  mucosa.  At  the  same  time,  too, 
the  uterine  tissues  surrounding  the  ovum  grow  up  around  it,  forming 
at  first  as  it  were  a  circular  wall  (Fig.  69,  A),  and  eventually  com- 

Sch. 


■-K- 


^,^^f^>r%^^        c  y 


iM^fe^ 


X,#' 


w 


Fig.  70. — Section  of  an  Ovum  of  i  mm.    A  Section  of  the  Embryo  Lies  in  the 

Lower  Part  of  the  Cavity  of  the  Ovum. 

D,  Decidua;  E.U.,  uterine  epithelium;  Sch,  blood-clot  closing  the  aperture  left  by 

the  sinking  of  the  ovum  into  the  uterine  mucosa. — (From  Strahl,  after  Peters.) 

pletely  enclose  it,  forming  an  envelope  known  as  the  decidua  cap- 
sularis  or  rejiexa.  The  blood  extravasation  is  now  contained  within 
a  closed  space  bounded  on  the  one  hand  by  the  uterine  tissues  and 
on  the  other  by  the  wall  of  the  ovum  (Fig.  69,  B). 

The  youngest  known  human  ova  have  already  reached  approxi- 


122  THE   CHORION 

mately  this  stage.  Thus,  the  Peters  ovum  (Fig.  70)  had  already 
sunk  deeply  into  the  uterine  mucosa,  the  point  of  entrance  being 
indicated  by  a  gap  in  the  decidua  capsularis,  closed  in  this  case  by  a 
patch  of  coagulated  blood  (Sch).  The  uterine  tissues  in  the  imme- 
diate vicinity  of  the  ovum  were  much  swollen  and  apparently  some- 
what necrotic  and  their  blood-vessels  could  be  seen  to  communicate 
with  the  space  between  the  wall  of  the  ovum  and  the  maternal  tissues. 
This  space,  however,  was  converted  into  an  irregular  network  of 
blood  lacunae  by  anastomosing  cords  of  cells,  which  arose  from  the 
wall  of  the  ovum  and  extended  through  the  space  to  the  maternal 
tissues ;  these  cords  of  cells  are  represented  in  Fig.  70  by  the  darker 
masses  projecting  from  the  wall  of  the  ovum  and  scattered  among 
the  paler  blood  lacunae.  This  stage  of  implantation  of  the  ovum  is 
shown  diagrammatically  in  Fig.  69,  B,  where,  for  simplicity's  sake, 
the  cell  cords  are  represented  merely  as  processes  radiating  from 
the  ovum  without  reaching  the  maternal  tissues. 

The  cell  cords  are  derivatives  of  the  trophoblast  and  are,  there- 
fore, of  embryonic  origin.  If  examined  under  a  higher  magnifica- 
tion than  that  shown  in  Fig.  70  they  will  be  seen  to  be  composed  of  an 
axial  core  of  cells  with  distinct  outlines,  enclosed  within  a  layer  of 
protoplasm  which  lacks  all  traces  of  cell  boundaries,  although  it 
contains  numerous  nuclei,  being  what  is  termed  a  syncytium  or 
Plasmodium.  The  original  trophoblast  has  thus  become  differen- 
tiated into  two  distinct  tissues,  a  cellular  one,  which  has  been  termed 
the  cyto-trophoblast,  and  a  plasmodial  one,  which,  similarly,  is 
known  as  the  plasmodi-trophoblast  and  is  the  tissue  that  comes  into 
contact  with  the  maternal  blood  contained  in  the  lacunar  spaces  and 
with  the  maternal  tissues,  in  connection  with  these  latter  sometimes 
developing  into  masses  of  considerable  extent.  To  this  plasmodi- 
trophoblast  may  be  ascribed  the  active  part  in  the  destruction  of 
the  maternal  tissues  and  probably  also  the  absorption  of  the  products 
of  the  destruction  for  the  nutrition  of  the  growing  ovum.  For  up  to 
this  stage  the  ovum  has  been  playing  the  role  of  a  parasite  thriving 
upon  the  tissues  of^  its  host. 

The  food  material  that  the  ovum  thus  obtains  may  conveniently 


THE    CHORION 


123 


be  termed  the  embryotroph  and  the  type  of  placentation  which  obtains 
up  to  this  stage  and  for  some  time  longer  may  be  termed  the  embryo- 
trophic  type.  But  even  in  the  Peters  ovum  the  preparation  for 
another  type  has  begun.  In  earlier  stages  the  cell  cords  were  entirely 
trophoblastic,  but  in  this  ovum  (Fig.  70)  processes  from  the  chorionic 
mesoderm  may  be  seen  projecting  into  the  bases  of  the  cell  cords, 
and  in  later  stages  these  processes  extend  farther  and  farther  into  the 
axis  of  each  cord,  the  anastomoses  of  the  cords  disappear  and  the 
cords  themselves  become  converted  into  branching  processes,  the 


Fig.   71. — Entire    Ovum    Aborted    at  about  the   Beginning  of  the   Second 
Month.     Xi  1/2. — (Grosser.) 


chorionic  villi,  which  project  from  the  entire  surface  of  the  ovum 
(Fig.  71)  into  the  surrounding  space,  which  may  now  be  termed  the 
intervillous  space,  and  are  bathed  by  the  maternal  blood  which  it 
contains.  Toward  the  maternal  surface  of  the  space  some  masses  of 
the  trophoblast  still  persist,  uniting  the  extremities  of  certain  of  the 
villi  to  the  enclosing  uterine  wall,  such  villi  being  termed  fixation 
villi  to  distinguish  them  from  the  majority,  which  project  freely  into 
the  intervillous  space.     Later,  when  the   embryonic  blood-vessels 


124 


THE   CHORION 


develop,  those  associated  with  the  allantois  extend  outward  into 
the  chorionic  mesoderm  and  thence  send  branches  into  each  villus. 
The  second  type  of  placentation,  the  hcemotrophic  type,  is  thus  estab- 
lished, the  fetal  blood  contained  in  the  vessels  of  the  villi  receiving 
nutrition  through  the  walls  of  the  villi  from  the  maternal  blood 
contained  in  the  intervillous  space,  and,  similarly,  transferring 
waste  products  to  it. 

At  first,  as  stated  above,  the  villi  usually  cover  the  entire  surface 
of  the  ovum,  but  later,  as  the  ovum  increases  in  size,  those  villi 
which  are  remote  from  the  attachment  of  the  belly-stalk  to  the  chorion 
are  placed  at  a  disadvantage  so  far  as  their  blood  supply  is  concerned 


Fig.  72. — Two  Villi  prom  the  Chorion  of  an  Embryo  of  7  mm. 

and  gradually  disappear,  and  this  process  continues  until,  finally, 
only  those  villi  are  retained  which  are  in  the  immediate  region  of 
the  belly-stalk  (Fig.  69,  C),  these  persisting  to  form  the  fetal  portion 
of  the  placenta.  By  these  changes  the  chorion  becomes  differenti- 
ated into  two  regions  (Fig.  69,  C),  one  of  which  is  destitute  of  villi  and 
is  termed  the  chorion  lave,  while  the  other  provided  with  them,  is 
known  as  the  chorion  frondosum. 


THE    CHORION 


1^5 


Fig.  73. — Transverse  Sections  through  Chorionic  Villi  in  (4)  the  Fifth 
and  (B)  the  Seventh  Month  of  Development. 

cf,  Canalized  fibrin;  Ic,  Langhans  cells;  s,  syncytium. — (A  which  is  more  highly 
magnified  than  B,  from  Szymonowicz;  B  from  Minot.) 


126  THE    CHORION 

Occasionally  one  or  more  patches  of  villi  may  persist  in  the  area  that 
normally  becomes  the  chorion  lseve  and  thus  accessory  placenta  {-placenta 
succenturiatce) ,  varying  in  number  and  size,  may  be  formed. 

The  villi  when  fully  formed  are  processes  of  the  chorion,  branch- 
ing profusely  and  irregularly  (Fig.  72),  and  each  consists  of  a  core  of 
mesoderm,  containing  blood-vessels,  enclosed  within  a  double 
layer  of  trophoblastic  tissue  (Fig.  73,  A).  The  inner  layer  consists 
of  a  sheet  of  well  defined  cells  arranged  in  a  single  series;  it  is 
derived  from  the  cyto-trophoblast  and  forms  what  is  known  as  the 
layer  of  Langhans  cells.  The  outer  layer  is  syncytial  in  structure 
and  is  formed  from  the  plasmodi-trophoblast. 


ck 


Fig.  74. — Mature  Placenta  after  Separation  from  the  Uterus. 
c,  Cotyledons;  eh,  chorion,  amnion,  and  decidua  vera;  urn,  umbilical  cord. — (Kollmann.) 

As  development  proceeds  the  villi,  which  are  at  first  distributed 
evenly  over  the  chorion  frondosum,  become  separated  into  groups 
termed  cotyledons  (Fig.  74)  by  the  growth  into  the  intervillous  space 
of  trabecular  from  the  walls  of  the  uterus,  the  fixation  villi  becoming 
connected  with  these  septa  as  well  as  with  the  general  uterine  wall. 
The  ectoderm  of  the  villi  also  undergoes  certain  changes  with  ad- 
vancing growth,  the  layer  of  Langhans  cells  disappearing  except  in 
small  areas  scattered  irregularly  in  the  villi,  and  the  syncytium, 


THE    CHORION 


127 


though  persisting,  undergoes  local  thickenings  which  become 
replaced,  more  or  less  extensively,  by  depositions  of  fibrin 
(Fig.  73,  B,  cf). 

The  changes  which  occur  during  the  later  stages  of  development 
in  the  chorion  are  very  similar  to  those  described  for  the  villi. 


TTIES 


y"B*3??r^^Bi"> 


Fig.  75. — Section  through  the  Placental  Chorion  of  an  Embryo  of  Seven 

Months. 
c,  Cell  layer;  ep,  remnants  of  epithelium;  fb,  fibrin  layer;  mes,  mesoderm. — {Minot.) 

Thus,  the  mesoderm  thickens,  its  outermost  layers  becoming 
exceedingly  fibrillar  in  structure,  while  the  ectoderm  differentiates 
into  two  layers,  the  outer  of  which  is  syncytial  while  the  inner  is 
cellular,  and  later  still,  as  in  the  villi,  the  syncytial  layer  is  replaced 


128  THE   DECIDED 

in  irregular  patches  by  a  peculiar  form  of  fibrin  which  is  traversed 
by  flattened  anastomosing  spaces  and  to  which  the  name  canalized 
fibrin  or  fibrinoid  has  been  applied  (Fig.  75). 

The  Deciduae. — It  has  been  pointed  out  (p.  26)  that  in  connec- 
tion with  the  phenomenon  of  menstruation  periodic  alterations 
occur  in  the  mucous  membrane  of  the  uterus.  If  during  one  of 
these  periods  a  fertilized  ovum  reaches  the  uterus,  the  desquamation 


Fig.  76. — Diagram  showing  the  Relations  of  the  Fetal  Membranes. 

Am,  Amnion;  Ch,  chorion;  M,  muscular  wall  of  uterus;  C,  decidua  capsularis;  B, 

decidua  basalis;  V,  decidua  vera;  F,  yolk-stalk. 

of  portions  of  the  epithelium  does  not  occur  nor  is  there  any  appre- 
ciable hemorrhage  into  the  cavity  of  the  uterus;  the  uterine  mucosa 
remains  in  what  is  practically  the  ante-menstrual  condition  until  the 
conclusion  of  pregnancy,  when,  after  the  birth  of  the  fetus,  a  con- 
siderable portion  of  its  thickness  is  expelled  from  the  uterus,  forming 
what  is  termed  the  decidua.     In  other  words,  the  sloughing  of  the 


THE   DECIDILE 


12$ 


uterine  tissue  which  concludes  the  process  of  menstruation  is  post- 
poned until  the  close  of  pregnancy,  and  then  takes  place  simultane- 
ously over  the  whole  extent  of  the  uterus.  Of  course,  the  changes 
in  the  uterine  tissues  are  somewhat  more  extensive  during  pregnancy 
than  during  menstruation,  but  there  is  an  undoubted  fundamental 
similarity  in  the  changes  during  the  two  processes. 


Fig.  77. — Surface  View  op  Half  of  the  Decldua  Vera  at  the  End  of  the  Third 

Week  of  Gestation. 

d,  Mucous  membrane  of  the  Fallopian  tubes;  ds,  prolongation  of  the  vera  toward  the 

cervix  uteri;  pp.,  papillae;  rf,  marginal  furrow.     (Kollmann.) 


The  human  ovum  comes  into  direct  apposition  with  only  a  small 
portion  of  the  uterine  wall,  and  the  changes  which  this  portion  of  the 
wall  undergoes  differ  somewhat  from  those  occurring  elsewhere. 
Consequently  it  becomes  possible  to  divide  the  deciduae  into  (1)  a 
portion  which  is  not  in  direct  contact  with  the  ovum,  the  decidua  vera 
(Fig.  76,  V)  and  (2)  a  portion  which  is.  The  latter  portion  is  again 
9 


13° 


THE   DECIDUA    VERA 


capable  of  division.  The  ovum  becomes  completely  embedded  in 
the  mucosa,  but,  as  has  been  pointed  out,  the  chorionic  villi  reach 
their  full  development  only  over  that  portion  of  the  chorion  to  which 
the  belly-stalk  is  attached.  The  decidua  which  is  in  relation  to  this 
chorion  frondosum  undergoes  much  more  extensive  modifications 
than  that  in  relation  to  the  chorion  laeve,  and 
to  it  the  name  of  decidua  basalts  (decidua 
serotina)  (Fig.  76,  B)  is  applied,  while  the 
rest  of  the  decidua  which  encloses  the  ovum 
is  termed  the  decidua  capsularis  (decidua 
rejlexa)  (C). 

The  changes  which  give  rise  to  the  decidua 
vera  may  first  be  described  and  those  occur- 
ring in  the  others  considered  in  succession. 

(a)  Decidua  vera. — On  opening  a  uterus 
during  the  fourth  or  fifth  month  of  pregnancy, 
when  the  decidua  vera  is  at  the  height  of  its 
development,  the  surface  of  the  mucosa  pre- 
sents a  corrugated  appearance  and  is  traversed 


Fig.  78. — Diagrammatic  Sections  of  the  Uterine   Mucosa,  A,  in  the  Non- 
pregnant Uterus,  and  B,  at  the  Beginning  of  Pregnancy. 
c,  Stratum  compactum;  gl,  the  deepest  portions  of  the  glands;  m,  muscular  layer; 
sp,  stratum  spongiosum. — (Kundrat  and  Engelmann.) 


by  irregular  and  rather  deep  grooves  (Fig.  77).  This  appearance 
ceases  at  the  internal  orifice,  the  mucous  membrane  of  the  cervix 
uteri  not  forming  a  decidua,  and  the  deciduae  of  the  two  surfaces  of 
the  uterus  are  separated  by  a  distinct  furrow  known  as  the  marginal 
groove. 


THE    DECIDUA    CAPSULARIS  131 

In  sections  the  mucosa  is  found  to  have  become  greatly  thick- 
ened, frequently  measuring  i  cm.  in  thickness,  and  its  glands  have 
undergone  very  considerable  modification.  Normally  almost 
straight  (Fig.  78,  A),  they  increase  in  length,  not  only  keeping  pace 
with  the  thickening  of  the  mucosa,  but  surpassing  its  growth,  so  that 
they  become  very  much  contorted  and  are,  in  addition,  considerably 
dilated  (Fig.  78,  B).  Near  their  mouths  they  are  dilated,  but  not 
very  much  contorted,  while  lower  down  the  reverse  is  the  case,  and 
it  is  possible  to  recognize  three  layers  in  the  decidua,  (1)  a  stratum 
compactum  nearest  the  lumen  of  the  uterus,  containing  the  straight 
but  dilated  portions  of  the  glands;  (2)  a  stratum  spongiosum,  so  called 
from  the  appearance  which  it  presents  in  sections  owing  to  the  dilated 
and  contorted  portions  of  the  glands  being  cut  in  various  planes; 
and  (3)  next  the  muscular  coat  of  the  uterus  a  layer  containing  the 
contorted  but  not  dilated  extremities  of  the  glands  is  found.  Only 
in  the  last  layer  does  the  epithelium  of  the  glands  retain  its  normal 
columnar  form;  elsewhere  the  cells,  separated  from  the  walls  of  the 
glands,  become  enlarged  and  irregular  in  shape  and  eventually 
degenerate. 

In  addition  to  these  changes,  the  epithelium  of  the  mucosa  disap- 
pears completely  during  the  first  month  of  pregnancy,  and  the 
tissue  between  the  glands  in  the  stratum  compactum  becomes  packed 
with  large,  often  multinucleated  cells,  which  are  termed  the  decidual 
cells  and  are  probably  derived  from  the  connective  tissue  cells  of  the 
mucosa. 

After  the  end  of  the  fifth  month  the  increasing  size  of  the  embryo 
and  its  membranes  exerts  a  certain  amount  of  pressure  on  the  decidua, 
and  it  begins  to  diminish'in  thickness.  The  portions  of  the  glands 
which  lie  in  the  stratum  compactum  become  more  and  more  com- 
pressed and  finally  disappear,  while  in  the  spongiosum  the  spaces 
become  much  flattened  and  the  vascularity  of  the  whole  decidua, 
at  first  so  pronounced,  diminishes  greatly. 

(b)  Decidua  capsularis. — The  decidua  capsularis  has  also  been 
termed  the  decidua  reflexa,  on  the  supposition  that  it  was  formed  as  a 
fold  of  the  uterine  mucosa  reflected  over  the  ovum  after  this  had 


132  THE   DECIDUA    BASALIS 

attached  itself  to  the  uterine  wall.  Since,  however,  the  attachment 
of  the  ovum  is  to  be  regarded  as  a  process  of  burrowing  into  the 
uterine  tissues  (see  p.  119),  the  necessity  for  an  upgrowth  of  a  fold  is 
limited  to  an  elevation  of  the  uterine  tissues  in  the  neighborhood  of 
the  ovum  to  keep  pace  with  its  increasing  size.  Since  it  is  part  of  the 
area  of  contact  with  the  ovum  it  possesses  no  epithelium  upon  the 
surface  turned  toward  the  ovum,  although  in  the  earlier  stages  its 
surface  is  covered  by  an  epithelium  continuous  with  that  of  the 
decidua  vera,  and  between  it  and  the  chorion  there  is  a  portion  of 
the  blood  extravasation  in  which  the  villi  formed  from  the  chorion 
laeve  float.  Glands  and  blood-vessels  also  occur  in  its  walls  in  the 
earlier  stages  of  development. 

As  the  ovum  continues  to  increase  in  size  the  capsularis  begins 
to  show  signs  of  degeneration,  these  appearing  first  over  the  pole 
of  the  ovum  opposite  the  point  of  fixation.  Here,  even  in  the  case 
of  the  ovum  described  by  Rossi  Doria,  the  cavity  of  which  measured 
6X5  mm.  in  diameter,  it  has  become  reduced  to  a  thin  membrane 
destitute  of  either  blood-vessels  or  glands,  and  the  degeneration 
gradually  extends  throughout  the  entire  capsule,  the  portion  of  the 
blood  space  which  it  encloses  also  disappearing.  At  about  the  fifth 
month  the  growth  of  the  ovum  has  brought  the  capsularis  in  contact 
throughout  its  whole  extent  with  the  vera,  and  it  then  appears  as  a 
whitish  transparent  membrane  with  ho  trace  of  either  glands  or 
blood-vessels,  and  it  eventually  disappears  by  fusing  with  the  vera. 

(c)  Decidua  basalis. — The  structure  of  the  decidua  basalis,  also 
known  as  the  decidua  serotina,  is  practically  the  same  as  that  of  the 
vera  up  to  about  the  fifth  month.  It  differs  only  in  that,  being  part  of 
the  area  of  contact  of  the  ovum,  it  loses  its  epithelium  much  earlier 
and  is  also  the  seat  of  extensive  blood  extravasations,  due  to  the 
erosion  of  its  vessels  by  the  chorionic  trophoblast.  Its  glands, 
however,  undergo  the  same  changes  as  those  of  the  vera,  so  that  in 
it  also  a  compactum  and  a  spongiosum  may  be  recognized.  Beyond 
the  fifth  month,  however,  there  is  a  great  difference  between  it  and 
the  vera,  in  that,  being  concerned  with  the  nutrition  of  the  embryo, 
it  does  not  partake  of  the  degeneration  noticeable  in  the  other  deciduae, 


THE    PLACENTA  133 

but  persists  until  birth,  forming  a  part  of  the  structure  termed  the 
placenta. 

The  Placenta. — This  organ,  which  forms  the  connection  between 
the  embryo  and  the  maternal  tissues,  is  composed  of  two  parts, 
separated  by  the  intervillous  space.  One  of  these  parts  is  of  embry- 
onic origin,  being  the  chorion  frondosum,  while  the  other  belongs  to 
the  maternal  tissues  and  is  the  decidua-  basalis.  Hence  the  terms 
placenta  fetalis  and  placenta  uterina  frequently  applied  to  the  two 
parts.  The  fully  formed  placenta  is  a  more  or  less  discoidal  struc- 
ture, convex  on  the  surface  next  the  uterine  muscularis  and  concave 
on  that  turned  toward  the  embryo,  the  umbilical  cord  being  continu- 
ous with  it  near  the  center  of  the  latter  surface.  It  averages  about 
3.5  cm.  in  thickness,  thinning  out  somewhat  toward  the  edges,  and 
has  a  diameter  of  15  to  20  cm.,  and  a  weight  varying  between  500 
and  1250  grams.  It  is  situated  on  one  of  the  surfaces  of  the  uterus, 
the  posterior  more  frequently  than  the  anterior,  and  usually  much 
nearer  the  fundus  than  the  internal  orifice.  It  develops,  in  fact, 
wherever  the  ovum  happens  to  become  attached  to  the  uterine  walls, 
and  occasionally  this  attachment  is  not  accomplished  until  the  ovum 
has  descended  nearly  to  the  internal  orifice,  in  which  case  the 
placenta  may  completely  close  this  opening  and  form  what  is  termed 
a  placenta  prcevia. 

If  a  section  of  a  placenta  in  a  somewhat  advanced  stage  of  develop- 
ment be  made,  the  following  structures  may  be  distinguished:  On 
the  inner  surface  there  will  be  a  delicate  layer  representing  the  amnion 
(Fig.  79,  Am),  and  next  to  this  a  somewhat  thicker  one  which  is  the 
chorion  (Cho),  in  which  the  degenerative  changes  already  mentioned 
may  be  observed.  Succeeding  this  comes  a  much  broader  area  com- 
posed of  the  large  intervillous  blood  space  in  which  lie  sections  of 
the  villi  (vi)  cut  in  various  directions.  Then  follows  the  stratum 
compactum  of  the  basalis,  next  the  stratum  spongiosum  (Z)')> 
next  the  outermost  layer  of  the  mucosa  (D"),  in  which  the 
uterine  glands  retain  their  epithelium,  and,  finally,  the  muscularis 
uteri    (Mc) 

These  various  structures  have,  for  the  most  part,  been  already 


134 


THE   PLACENTA 


Fig.  79. — Section  through  a  Placenta  of  Seven  Months'  Development. 

Am,  Amnion;  cho,  chorion;  D,  layer  of  decidua  containing  the  uterine  glands ;{Mc, 
muscular  coat  of  the  uterus;  Ve,  maternal  blood-vessel;  Vi,  stalk  of  a  villus;  vi,  villi 
in  section. — (Minoi.) 


THE    PLACENTA  135 

described  and  it  remains  here  only  to  say  a  few  words  concerning  the 
special  structure  of  the  basal  compactum  and  concerning  certain 
changes  that  take  place  in  the  intervillous  space. 

The  stratum  compactum  of  the  basal  decidua  forms  what  is 
termed  the  basal  plate  of  the  placenta,  closing  the  intervillous  space 
on  the  uterine  side  and  being  traversed  by  the  maternal  blood-vessels 
that  open  into  the  space.  The  formation  of  canalized  fibrin,  already 
mentioned  in  connection  with  the  decidua  vera  and  the  syncytium  of 
the  villi,  also  occurs  in  the  basal  portion  of  the  decidua,  a  definite 
layer  of  it,  known  as  NitabucJi's  fibrin  stria,  being  a  characteristic 
constituent  of  the  basal  plate  and  patches  of  greater  or  less  extent 
also  occur  upon  the  surface  of  the  plate.  Leucocytes  also  occur  in 
considerable  abundance  in  the  plate  and  their  presence  has  been 
taken  to  indicate  an  attempt  on  the  part  of  the  maternal  tissues  to 
resist  the  erosive  action  of  the  parasitic  ovum.  From  the  surface 
of  the  basal  plate  processes,  termed  placental  septa,  project  into  the 
intervillous  space,  grouping  the  villi  into  cotyledons  and  giving 
attachment  to  some  of  the  fixation  villi  (Fig.  80).  Throughout  the 
greater  extent  of  the  placenta  the  septa  do  not  reach  the  surface  of 
the  chorion,  but  at  the  periphery,  throughout  a  narrow  zone,  they 
do  come  into  contact  with  the  chorion  and  unite  beneath  it  to  form  a 
membrane  which  has  been  termed  the  closing  plate.  Beneath  this  lies 
the  peripheral  portion  of  the  intervillous  space,  which,  owing  to  the 
arrangement  of  the  septa  in  this  region,  appears  to  be  imperfectly 
separated  from  the  rest  of  the  space  and  forms  what  is  termed  the 
marginal  sinus  (Fig.  80). 

Attention  has  already  been  called  to  the  formation  of  canalized 
fibrin  or  fibrinoid  in  connection  with  the  syncytium  of  the  villi.  In 
the  later  stages  of  pregnancy  there  may  be  produced  by  this  process 
masses  of  fibrinoid  of  considerable  size,  lying  in  the  intervillous  space; 
these,  on  account  of  their  color,  are  termed  white  infarcts  and  may 
frequently  be  observed  as  whitish  or  grayish  patches  through  the 
walls  of  the  placenta  after  its  expulsion.  Red  infarcts  produced  by 
the  clotting  of  the  blood,  also  occurs,  but  with  much  less  regularity 
and  frequency. 


136 


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SEPARATION    OF    TEE   DECTDtLE  137 

The  Separation  of  the  Deciduae  at  Birth. — At  parturition, 
after  the  rupture  of  the  amnion  and  the  expulsion  of  the  fetus,  there 
still  remain  in  the  uterine  cavity  the  deciduae  and  the  amnion,  which 
is  in  contact  but  not  fused  with  the  deciduae.  A  continuance  of  the 
uterine  contractions,  producing  what  are  termed  the  "after-pains," 
results  in  the  separation  of  the  placenta  from  the  uterine  walls,  the 
separation  taking  place  in  the  deep  layers  of  the  spongiosum,  so 
that  the  portion  of  the  mucosum  which  contains  the  undegenerated 
glands  remains  behind.  As  soon  as  the  placenta  has  separated, 
the  separation  of  the  decidua  vera  takes  place  gradually  though 
rapidly,  the  line  of  separation  again  being  in  the  deeper  layers  of  the 
stratum  spongiosum,  and  the  whole  of  the  deciduae,  together  with 
the  amnion,  is  expelled  from  the  uterus,  forming  what  is  known  as 
the  "after-birth." 

Hemorrhage  from  the  uterine  vessels  during  and  after  the  separa- 
tion of  the  deciduae  is  prevented  by  the  contractions  of  the  uterine 
walls,  assisted,  according  to  some  authors,  by  a  preliminary  blocking 
of  the  mouths  of  the  uterine  vessels  by  certain  large  polynuclear 
decidual  cells  found  during  the  later  months  of  pregnancy  in  the  outer 
layers  of  the  decidua  basalis.  The  regeneration  of  the  uterine  mucosa 
after  parturition  has  its  starting-point  from  the  epithelium  of  the 
undegenerated  glands  which  persist,  this  epithelium  rapidly  evolving 
a  complete  mucosa  over  the  entire  surface  of  the  uterus. 

The  complicated  arrangement  of  the  human  placenta  is,  of  course, 
the  culmination  of  a  long  series  of  specializations,  the  path  along  which 
these  have  proceeded  being  probably  indicated  by  the  conditions  obtaining 
in  some  of  the  lower  mammals.  The  Monotremes  resemble  the  reptiles 
in  being  oviparous  and  in  this  group  of  forms  there  is  no  relation  of  the 
ovum  to  the  maternal  tissues  such  as  occurs  in  the  formation  of  a  placenta. 
In  the  other  mammals  viviparity  is  the  rule  and  this  condition  does 
demand  some  sort  of  connection  between  the  fetal  and  maternal  tissues. 
One  of  the  simplest  of  such  connections  is  that  seen  in  the  pig,  where  the 
chorionic  villi  of  the  ovum  fit  into  corresponding  depressions  in  the 
uterine  mucosa,  this  tissue,  however,  undergoing  no  destruction,  and  at 
birth  the  villi  simply  withdraw  from  the  depressions  of  the  mucosa, 
leaving  it  intact.  This  type  of  placentation  is  an  embryo  trophic  one,  and 
since  there  is  no  separation  of  deciduae  from  the  uterine  wall  after  preg- 
nancy it  is  also  of  the  indeciduate  type.     In  the  sheep  the  placentation  is 


138  LITERATURE 

also  embryotrophic  and  indeciduate,  but  destruction  of  the  maternal 
mucosa  does  take  place,  the  villi  penetrating  deeply  into  it  and  coming  into 
relation  with  the  connective  tissue  surrounding  the  maternal  blood-vessels. 
Another  step  in  advance  is  shown  by  the  dog,  in  which  even  the  con- 
nective tissue  around  the  maternal  vessels  in  the  placental  area  undergoes 
almost  complete  destruction  so  that  the  chorionic  villi  are  separated 
from  the  maternal  blood  practically  only  by  the  endothelial  lining  of  the 
maternal  vessels.  In  this  case  the  mucosa  undergoes  so  much  alteration 
that  the  undestroyed  portions  if  it  are  sloughed  off  after  birth  as  a  decidua, 
so  that  the  placentation,  like  that  in  man,  is  of  the  deciduate  type.  It 
still  represents,  however,  an  embryotrophic  type,  although  closely  approxi- 
mating to  the  haemotrophic  one  found  in  man,  in  which,  as  described  above, 
the  destruction  of  the  maternal  tissues  proceeds  so  far  as  to  open  into  the 
maternal  blood-vessels,  so  that  the  fetal  villi  are  in  direct  contact  with  the 
maternal  blood. 

If  these  various  stages  may  be  taken  to  represent  steps  by  which 
the  conditions  obtaining  in  the  human  placenta  have  been  evolved,  the 
entire  process  may  be  regarded  as  the  result  of  a  progressive  activity  of  a 
parasitic  ovum.  In  the  simplest  stage  the  pabulum  supplied  by  the 
uterus  was  sufficient  for  the  nutrition  of  the  parasite,  but  gradually  the 
ovum,  by  means  of  its  plasmodi-trophoblast,  began  to  attack  the  tissues 
of  its  host,  thus  obtaining  increased  nutrition,  until  finally,  breaking 
through  into  the  maternal  blood-vessels,  it  achieved  for  itself  still  more 
favorable  nutrition,  by  coming  into  direct  contact  with  the  maternal 
blood. 

LITERATURE. 

In  addition  to  the  papers  by  Beneke  and  Strahl,  Bryce  and  Teacher,  Frassi,  Jung, 
and  Herzog,  cited  in  Chapter  III,  the  following  may  be  mentioned: 

E.  Cova:  "  Ueber  ein  menschliches  Ei  der  zweiten  Woche,"  Arch,  fur  Gynaek.,  lxxxiii, 

1907. 
L.  Frassi:  "Ueber  ein  junges  menschliches  Ei  in  situ,"  Arch,  fiir  mikr.  Anal.,  lxx, 

1907. 
O.   Grosser:  "Vergleichende   Anatomic   und   Entwicklungsgeschichte   der   Eihaute 

und  der  Placenta  mit  besonderer  Berticksichtigung  des  Menschen,"  Wien,  1909. 
H.  Happe:  "Beobachtungen  an  Eihauten    junger  menschlicher  Eier,"  Anat.  Hefte, 

xxxii,  1906. 
W.  His:  "Die  Umschliessung  der  menschlichen  Frucht  wahrend  der  friihesten  Zeit. 

des  Schwangerschafts,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1897. 
M.  Hofmeier:  "Die  menschliche  Placenta,"  Wiesbaden,  1890. 

F.  Keibel:  "Zur  Entwickelungsgeschichte  der  Placenta,"  Anat.  Anzeiger,  iv,  1889. 

J.  Kollmann:  "Die  menschlichen  Eier  von  6  mm.  Grosse,"  Archiv  fiir  Anat.  und 
Physiol.,  Anat.  Abth.,  1879. 

G.  Leopold:  "Ueber  ein  sehr  junges  menschliches  Ei  in  situ,"  Arb.  aus  der 

Frauenklinik  in  Dresden,  rv,  1906. 


LITERATURE  139 

F.  Marchand:  "Beobachtungen  an  jungen  menschlichen  Eiern,"  Anat.Hefte,  xxi, 

1903. 
J.  Merttens:  "Beitrage  zur  normalen  und  pathologischen  Anatomie  der  mensch- 

lichen  Placenta,"  Zeitschrift  fiir  Geburtshiilfe  und  Gynaekol.,  xxx  and  xxxi,   1894. 
C.  S.  Minot:  "Uterus  and  Embryo,"  Journal  of  Morphol.,  n,  1889. 

G.  Paladino:  "Sur  la  genese  des  espaces  intervilleux  du  placenta  humain  et  de  leur 

premier  contenu,  comparativement  a  la  meme  partie  chez  quelques  mammiferes," 

Archives  Ital.  de  Biolog.,  xxxi  and  xxxn,  1899. 
H.  Peters:  "Ueber  die  Einbettung  des  menschlichen  Eies  und  das  friiheste  bisher 

bekannte  menschliche  Placentationsstadium,"  Leipzig  und  Wien,  1899. 
J.  Rejsek:  "Anheftung  (Implantation)  des  Sangetiereies  an  die  Uteruswand,  insbe- 

sondere  des  Eies  von  Spermophilus  citellus,"  Arch,  fiir  mikrosk.  Anat.,  lxiii,  1964. 
T.  Rossi  Doria:  "Ueber  die  Einbettung  des  menschlichen  Eies,  studirt  an  einem 

kleinen  Eie  der  zweiten  Woche,"  Arch,  fiir  Gynaek.,  lxxvi.  1905. 
C.    Ruge:  "Ueber  die   menschliche  Placentation,"  Zeitschrift  fur  Geburtshiilfe  und 

Gynaekol.,  xxxix,  1898. 
Siegenbeek  van  Hetjkelom:  "Ueber  die  menschliche  Placentation,"  Arch.  f.  Anat. 

undPhys.,  Anat.  Abth.,  1898. 
F.  Graf  Spee:  "Ueber  die  menschliche  Eikammer  und  Decidua  reflexa,"  Verhandl. 

des  Anat.  Gesellsch.,  xii,  1898. 
H.  Strahl:  "Die  menschliche  Placenta,"  Ergebn  der  Anat.  und  Enlwickl.,  II,  1893. 

"Neues  uber  den  Bau  der  Placenta,"  ibid,  vi,  1897. 

"Placentaranatomie,"  ibid.,  viii,  1899. 
R.  Todyo:  "Ein  junges  menschliches  Ei,"  Arch,  fiir  Gynaek.,  xcv,  1912. 
Van   Cauwenberghe  :   "Recherches   sur   la   role   du   Syncytium  dans   la   nutrition 

embryonnaire  de  la  femme,"  Arch,  de  Biol.,  xxiii,  1907. 
J.  C.  Webster:  "Human  Placentation,"  Chicago,  1901. 
E.  Wormser:    "Die  Regeneration  der  Uterusschleimhaut  nach  der  Geburt,"  Arch. 

fiir  Gynaek.,  lxix,  1903. 


PART  II. 
ORGANOGENY. 


CHAPTER  VI. 
THE  DEVELOPMENT  OF  THE  INTEGUMENTARY  SYSTEM. 

The  Development  of  the  Skin. — The  skin  is  composed  of  two 
embryologically  distinct  portions,  the  outer  epidermal  layer  being 
developed  from  the  ectoderm,  while  the  dermal  layer  is  mesen- 
chymatous  in  its  origin. 

The  ectoderm  covering  the  general  surface  of  the  body  is,  in  the 
earliest  stages  of  development,  a  single  layer  of  cells,  but  at  the  end 
of  the  first  month  it  is  composed  of  two  layers,  an  outer  one,  the 
epitrichium,  consisting  of  slightly  flattened  cells,  and  a  lower  one 
whose  cells  are  larger  and  which  will  give  rise  to  the  epidermis 
(Fig.  81,  A).  During  the  second  month  the  differences  between 
the  two  layers  become  more  pronounced,  the  epitrichial  cells  assum- 
ing a  characteristic  domed  form  and  becoming  vesicular  in  structure 
(Fig.  81,  B).  These  cells  persist  until  about  the  sixth  month  of 
development,  but  after  that  they  are  cast  off,  and,  becoming  mixed 
with  the  secretion  of  sebaceous  glands  which  have  appeared  by  this 
time,  form  a  constituent  of  the  vernix  caseosa. 

In  the  meantime  changes  have  been  taking  place  in  the  epidermal 
layer  which  result  in  its  becoming  several  layers  thick  (Fig.  81,  B), 
the  innermost  layer  being  composed  of  cells  rich  in  protoplasm, 
while  those  of  the  outer  layers  are  irregular  in  shape  and  have  clearer 
contents.  As  development  proceeds  the  number  of  layers  increases 
and  the  superficial  ones,  undergoing  a  horny  degeneration,  give  rise 
to  the  stratum  corneum,  while  the  deeper  ones  become  the  stratum 

141 


142 


DEVELOPMENT    OF    THE    SKIN 


Malpighii.  At  about  the  fourth  month  ridges  develop  on  the  under 
surface  of  the  epidermis,  projecting  downward  into  the  dermis, 
and  later  secondary  ridges  appear  in  the  intervals  between  the 
primary  ones,  while  on  the  palms  and  soles  ridges  appear  upon  the 
outer  surface  of  the  epidermis,  corresponding  in  position  to  the 
primary  ridges  of  the  under  surface. 

The  mesenchyme  which  gives  rise  to  the  dermis  grows  in  from 
all  sides  between  the  epidermis  and  the  outer  layer  of  the  myotomes, 


si^j^mmw** 


Fig.  81. — A,  Section  of  Skin  from  the  Dorsum  of  Finger  of  an  Embryo  of  4.5  cm.; 

B,  from  the  Plantar  Surface  of  the  Foot  of  an  Embryo  of  10.2  cm 

et,  Epitrichium;  ep,  epidermis. 


which  are  at  first  in  contact,  and  forms  a  continuous  layer  under- 
lying the  epidermis  and  showing  no  indications  of  a  segmental 
arrangement.  It  becomes  converted  "principally  into  fibrous  con- 
nective tissue,  the  outer  layers  of  which  are  relatively  compact, 
while  the  deeper  ones  are  looser,  forming  the  subcutaneous  areolar 
tissue.  Some  of  the  mesenchymal  cells,  however,  become  converted 
into  non-striated  muscle-fibers,  which  for  the  most  part  are  few  in 
number  and  associated  with  the  hair  follicles,  though  in  certain 
regions,  such  as  the  skin  of  the  scrotum,  they  are  very  numerous  and 


DEVELOPMENT    OF    THE    SKIN 


form  a  distinct  layer  known  as  the  dartos. 
Some  cells  also  arrange  themselves  in  groups 
and  undergo  a  fatty  degeneration,  well-defined 
masses  of  adipose  tissue  embedded  in  the 
lower  layers  of  the  dermis  being  thus  formed 
at  about  the  sixth  month. 

Although  the  dermal  mesenchyme  is  unseg- 
mental  in  character,  yet  the  nerves  which  send 
branches  to  it  are  segmental,  and  it  might  be 
expected  that  indications  of  this  condition  would 
be  retained  by  the  cutaneous  nerves  even  in  the 
adult.  A  study  of  the  cutaneous  nerve-supply  in 
the  adult  realizes  to  a  very  considerable  extent 
this  expectation,  the  areas  supplied  by  the  various 
nerves  forming  more  or  less  distinct  zones,  and 
being  therefore  segmental  (Fig.  82).  But  a  con- 
siderable commingling  of  adjacent  areas  has  also 
occurred.  Thus,  while  the  distribution  of  the 
cutaneous  branches  of  the  fourth  thoracic  nerve, 
as  determined  experimentally  in  the  monkey 
(Macacus),  is  distinctly  zonal  or  segmental,  the 
nipple  lying  practically  in  the  middle  line  of  the 
zone,  the  upper  half  of  its  area  is  also  supplied  or 
overlapped  by  fibers  of  the  third  nerve  and  the 
lower  half  by  fibers  of  the  fifth  (Fig.  83),  so  that 
any  area  of  skin  in  the  zone  is  innervated  by  fibers 
coming  from  at  least  two  segmental  nerves  (Sher- 
rington). And,  furthermore,  the  distribution  of 
each  nerve  crosses  the  mid-ventral  line  of  the  body, 
forming  a  more  or  less  extensive  crossed  overlap. 

And  not  only  is  there  a  confusion  of  adjacent 
areas  but  an  area  may  shift  its  position  relatively 
to  the  deeper  structures  supplied  by  the  same 
nerve,  so  that  the  skin  over  a  certain  muscle  is  not 
necessarily  supplied  by  fibers  from  the  nerve 
which  supplies  the  muscle.  Thus,  in  the  lower 
half  of  the  abdomen,  the  skin  at  any  point  will 
be  supplied  by  fibers  from  higher  nerves  than 
those  supplying  the  underlying  muscles  (Sherring- 
ton), and  the  skin  of  the  limbs  may  receive  twigs 
from  nerves  which  are  not  represented  at  all  in 
the  muscle-supply  (second  and  third  thoracic  and 
third  sacral). 


Ti 


'Ts  7i\ 


^ 


Ts 


Te 


Ja 


Ts 


r,o 


S2 


Lj 


IJ 


Sj 

Fig.  82. — Diagram 
showing  the  cutane- 
OUS Distribution  of  the 
Spinal  Nerves— (Head.) 


144 


DEVELOPMENT   OF   THE   NAILS 


The  Development  of  the  Nails.— The  earliest  indications  of 
the  development  of  the  nails  have  been  described  by  Zander  in 
embryos  of  about  nine  weeks  as  slight  thickenings  of  the  epidermis 


Fig.  83. — Diagram  showing  the  Overlap  of  the  III,  IV,  and  V  Intercostal 
Nerves  of  a  Monkey. — (Sherrington.) 


Fig.  84. — Longitudinal  Section  through  the  Terminal  Joint  of  the  Index- 
Finger  of  an  Embryo  of  4.5  cm. 
e,  Epidermis;  ep,  epitrichium;  nf,  nail  fold;  Ph,  terminal  phalanx;  sp,  sole  plate. 

of  the  tips  of  the  digits,  these  thickenings  being  separated  from  the 
neighboring  tissue  by  a  faint  groove.  Later  the  nail  areas  migrate 
to  the  dorsal  surfaces  of  the  terminal  phalanges  (Fig.  84)  and  the 


DEVELOPMENT    OF    THE    NAILS 


145 


sp- 


sc 


ep 


grooves  surrounding  the  areas  deepen,  especially  at  their  proximal 
edges,  where  they  form  the  nail-folds  (nf) ,  while  distally  thickenings 
of  the  epidermis  occur  to  form  what  have  been 
termed  sole-plates  (sp),  structures  quite  rudi- 
mentary in  man,  but  largely  developed  in  the 
lower  animals,  in  which  they  form  a  considerable 
portion  of  the  claws. 

The  actual  nail  substance  does  not  form, 
however,  until  the  embryo  has  reached  a  length 
of  about  17  cm.  By  this  time  the  epidermis  has 
become  several  layers  thick  and  its  outer  layers, 
over  the  nail  areas  as  well  as  elsewhere,  have 
become  transformed  into  the  stratum  corneum 
(Fig.  85,  sc),  and  it  is  in  the  deep  layers  of  this 
(the  stratum  lucidum)  that  keratin  granules  de- 
velop in  cells  which  degenerate  to  give  rise  to 
the  nail  substance  (n).  At  its  first  formation, 
accordingly,  the  nail  is  covered  by  the  outer  layers 
of  the  stratum  corneum  as  well  as  by  the  epi- 
trichium,  the  two  together  forming  what  has 
been  termed  the  eponychium  (Fig.  85,  ep).  The 
epitrichium  soon  disappears,  however,  leaving 
only  the  outer  layers  of  the  stratum  corneum  as 
a  covering,  and  this  also  later  disappears  with  the 
exception  of  a  narrow  band  surrounding  the  base 
of  the  nail  which  persists  as  the  perionyx. 

The  formation  of  the  nail  begins  in  the  more 
proximal  portion  of  the  nail  area  and  its  further 
growth  is  by  the  addition  of  new  keratinized 
cells  to  its  proximal  edge  and  lower  surface, 
these  cells  being  formed  only  in  the  proximal  part 
of  the  nail  bed  in  a  region  marked  by  its  whitish 
color  and  termed  the  lunula. 


The  first  appearance  of  the  nail-areas  at  the  tips 
of  the  digits  as  described  by  Zander  has  not  yet  been 


Fig.  85. — Longi- 
tudinal Section 
through  the  nail 
Area  in  an  Embryo 

OF  17  CM. 

ep,  Eponychium; 
n,  nail  substance;  nb, 
nail  bed;  sc,  stratum 
corneum;  sp,  sole 
plate. — (Okamura.) 


146 


DEVELOPMENT    OF    THE   HAIRS 


confirmed  by  later  observers,  but  the  migration  of  the  areas  to  the  dorsal 
surface  necessitated  by  such  a  location  of  the  primary  differentiation  affords 
an  explanation  of  the  otherwise  anomalous  cutaneous  nerve-supply  of  the 
nail-areas  in  the  adult,  this  being  from  the  palmar  (plantar)  nerves. 

The  Development  of  the  Hairs. — The  hairs  begin  to  develop 
at  about  the  third  month  and  continue  to  be  formed  during  the 
remaining  portions  of  fetal  life.  They  arise  as  solid  cylindrical 
downgrowths,  projecting  obliquely  into  the  subjacent  dermis  from 


to  -wl^lvfi 


p  ... 


A 


Fig.  86. — The  Development  of  a  Hair. 
c,  Cylindrical  cells  of  stratum  mucosum;  hf,  wall  of  hair  follicle;  m,  mesoderm; 
mu,  stratum  mucosum  of  epidermis;  p,  hair  papilla;  r,  root  of  hair;  s,  sebaceous  gland. 
— (Kollmann.) 

the  lower  surface  of  the  epidermis.  As  these  downgrowths  continue 
to  elongate,  they  assume  a  somewhat  club-shaped  form  (Fig.  86,  A), 
and  later  the  extremity  of  each  club  moulds  itself  over  the  summit  of 
a  small  papilla  which  develops  from  the  dermis  (Fig.  86,  B).  Even 
before  the  dermal  papilla  has  made  its  appearance,  however,  a 


DEVELOPMENT    OF    THE   HAIRS  147 

differentiation  of  the  cells  of  the  downgrowth  becomes  evident,  the 
central  cells  becoming  at  first  spindle-shaped  and  then  undergoing 
a  keratinization  to  form  the  hair  shaft,  while  the  more  peripheral 
ones  assume  a  cuboidal  form  and  constitute  the  lining  of  the  hair 
follicle.  The  further  growth  of  the  hair  takes  place  by  the  addi- 
tion to  its  basal  portion  of  new  keratinized  cells,  probably  produced 
by  the  multiplication  of  the  epidermal  cells  which  envelop  the 
papilla. 

From  the  cells  which  form  the  lining  of  each  follicle  an  outgrowth 
takes  place  into  the  surrounding  dermis  to  form  a  sebaceous  gland, 
which  is  at  first  solid  and  club-shaped,  though  later  it  becomes 
lobed.  The  central  cells  of  the  outgrowth  separate  from  the  per- 
ipheral and  from  one  another,  and,  their  protoplasm  undergoing  a 
fatty  degeneration,  they  finally  pass  out  into  the  space  between  the 
follicle  walls  and  the  hair  and  so  reach  the  surface,  the  peripheral 
cells  later  giving  rise  by  division  to  new  generations  of  central  cells. 
During  fetal  life  the  fatty  material  thus  poured  out  upon  the  surface 
of  the  body  becomes  mingled  with  the  cast-off  epitrichial  cells  and 
constitutes  the  white  oleaginous  substance,  the  vernix  caseosa,  which 
covers  the  surface  of  the  new-born  child.  The  muscles,  arrectores 
pilorum,  connected  with  the  hair  follicles  arise  from  the  mesen- 
chyme cells  of  the  surrounding  dermis. 

The  first  growth  of  hairs  forms  a  dense  covering  over  the  entire 
surface  of  the  fetus,  the  hairs  which  compose  it  being  exceedingly  fine 
and  silky  and  constituting  what  is  termed  the  lanugo.  This  growth 
is  cast  off  soon  after  birth,  except  over  the  face,  where  it  is  hardly 
noticeable  on  account  of  its  extreme  fineness  and  lack  of  coloration. 
The  coarser  hairs  which  replace  it  in  certain  regions  of  the  body 
probably  arise  from  new  follicles,  since  the  formation  of  follicles  takes 
place  throughout  the  later  periods  of  fetal  life  and  possibly  after 
birth.  But  even  these  later  formed  hairs  do  not  individually  persist 
for  any  great  length  of  time,  but  are  continually  being  shed,  new  or 
secondary  hairs  normally  developing  in  their  places.  The  shedding 
of  a  hair  is  preceded  by  a  cessation  of  the  proliferation  of  the  cells 
covering  the  dermal   papilla  and  by  a  shrinkage  of  the   papilla, 


148 


DEVELOPMENT   OF   THE   SUDORIPAROUS    GLANDS 


~h 


whereby  it  becomes  detached  from  the  hair,  and  the  replacing  hair 
arises  from  a  papilla  which  is  probably  budded  off  from  the  older 
one  before  its  degeneration  and  carries  with  it  a  cap  of  epidermal 
cells. 

It  is  uncertain  whether  the  cases  of  excessive  development  of  hair 
over  the  face  and  upper  part  of  the  body  which  occasionally  occur  are 
due  to  an  excessive  development  of  the  later  hair  follicles  (hypertrichosis) 
or  to  a  persistence  and  continued  growth  of  the  lanugo. 

The  Development  of  the  Sudoriparous  Glands. — The  sudor- 
iparous glands  arise  during  the  fifth  month  as  solid  cylindrical  out- 
growths from  the  primary  ridges 
of  the  epidermis  (Fig.  87),  and 
at  first  project  vertically  down- 
ward into  the  subjacent  dermis. 
Later,  however,  the  lower  end  of 
each  downgrowth  is  thrown  into 
coils,  and  at  the  same  time  a 
lumen  appears  in  the  center. 
Since,  however,  the  cylinders  are 
formed  from  the  deeper  layers 
of  the  epidermis,  their  lumina  do 
not  at  first  open  upon  the  sur- 
face, but  gradually  approach  it 
as  the  cells  of  the  deeper  layers 
of  the  epidermis  replace  those  which  are  continually  being  cast  off 
from  the  surface  of  the  stratum  corneum.  The  final  opening  to 
the  surface  occurs  during  the  seventh  month  of  development. 

The  Development  of  the  Mammary  Glands. — In  the  majority 
of  the  lower  mammals  a  number  of  mammary  glands  occur,  ar- 
ranged in  two  longitudinal  rows,  and  it  has  been  observed  that  in  the 
pig  the  first  indication  of  their  development  is  seen  in  a  thickening 
of  the  epidermis  along  a  line  situated  at  the  junction  of  the  abdomi- 
nal walls  with  the  membrana  reuniens  (Schulze).  This  thickening 
subsequently  becomes  a  pronounced  ridge,  the  milk  ridge,  from 
which,  at  certain  points,  the  mammary  glands  develop,  the  ridge 


Fig.  87. — Lower  Surface  of  a  De- 
tached Portion  of  Epidermis  from 
the  Dorsum  of  the  Hand. 
h,  Hair  follicle;  s,  sudoriparous  gland. — 
(Blaschko.) 


DEVELOPMENT    OF    THE    MAMMARY    GLANDS 


149 


disappearing  in  the  intervals.  In  a  human  embryo  4  mm.  in  length 
an  epidermal  thickening  has  been  observed  which  extended  from 
just  below  the  axilla  to  the  inguinal  region  (Fig.  88)  and  was  appar- 
ently equivalent  to  the  milk  line  of  the  pig,  and  in  embryos  of  14  or 
15  mm.  the  upper  end  of  the  line  had  become  a  pronounced  ridge, 
while  more  posteriorly  the  thickening  had  disappeared. 

The  further  history  of  the  ridge  has  not,  however,  been  yet 
traced  in  human  embryos,  and  the  next  stage  of  the  development  of 
the  glands  which  has  been  ob- 
served is  one  in  which  they  are 
represented  by  a  circular  thick- 
ening of  the  epidermis  which 
projects  downward  into  the 
dermis  (Fig.  89,  A).  Later 
the  thickening  becomes  lobed 
(Fig.  89,  B),  and  its  superficial 
and  central  cells  become  corni- 
fied  and  are  cast  off,  so  that  the 
gland  area  appears  as  a  depres- 
sion of  the  surface  of  the  skin. 
During  the  fifth  and  sixth 
months  the  lobes  elongate  into 
solid  cylindrical  columns  of  cells 
(Fig.  90)  resembling  not  a  little  the  cylinders  which  become  con- 
verted into  sudoriparous  glands,  and  each  column  becomes  slightly 
enlarged  at  its  lower  end,  from  which  outgrowths  begin  to  develop  to 
form  the  acini.  A  lumen  first  appears  in  the  lower  ends  of  the  col- 
umns and  is  formed  by  the  separation  and  breaking  down  of  the 
central  cells,  the  peripheral  cells  persisting  as  the  lining  of  the  acini 
and  ducts. 

The  elevation  of  the  gland  area  above  the  surface  to  form  the 
nipple  appears  to  occur  at  different  periods  in  different  embryos  and 
frequently  does  not  take  place  until  after  birth.  In  the  region  around 
the  nipple  sudoriparous  and  sebaceous  glands  develop,  the  latter 
also  occurring  within  the  nipple  area  and  frequently  opening  into 


Fig.  88. — Milk  Ridge  (mr)  in  a  Human 
Embryo. — (Kallius.) 


150  DEVELOPMENT    OF    THE    MAMMARY    GLANDS 

the  extremities  of  the  lacteal  ducts.  In  the  areola,  as  the  area  sur- 
rounding the  nipple  is  termed,  other  glands  known  as  Montgomery' 's 
glands,  also  appear,  their  development  resembling  that  of  the  mam- 
mary gland  so  closely  as  to  render  it  probable  that  they  are  really 
rudimentary  mammary  glands. 


J 


K»' 


"  B 

Fig.  89. — Sections  through  the  Epidermal  Thickenings  which  Represent  the 
Mammary  Gland  in  Embryos  (A)  of  6  cm.  and  (B)  or  10.2  cm. 


The  further  development  of  the  glands,  consisting  of  an  increase 
in  the  length  of  the  ducts  and  the  development  from  them  of  addi- 
tional acini,  continues  slowly  up  to  the  time  of  puberty  in  both  sexes, 
but  at  that  period  further  growth  ceases  in  the  male,  while  in  females 
it  continues  for  a  time  and  the  subjacent  dermal  tissues,  especially 
the  adipose  tissue,  undergo  a  rapid  development. 


LITERATURE  151 

The  occurrence  of  a  milk  ridge  has  not  yet  been  observed  in  a  sufficient 
number  of  embryos  to  determine  whether  it  is  a  normal  development  or 
is  associated  with  the  formation  of  supernumerary  glands  {polymastia). 
This  is  by  no  means  an  infrequent  anomaly;  it  has  been  observed  in  19 
per  cent,  of  over  100,000  soldiers  of  the  German  army  who  were  examined, 
and  occurs  in  47  per  cent,  of  individuals  in  certain  regions  of  Germany 
The  extent  to  which  the  anomaly  is  developed  varies  from  the  occurrence 
of  well-developed  accessory  glands  to  that  of  rudimentary  accessory 
nipples  {Jiy perihelia),  these  latter  sometimes  occurring  in  the  areolar  area 
of  a  normal  gland  and  being  possibly  due  in  such  cases  to  an  hypertrophy 
of  one  or  more  of  Montgomery's  glands. 


c€;   *  .-/'-* .,  °     '>_, 

Fig.  90. — Section  through  the  Mammary   Gland   of  an  Embryo  of  25  cm. 
1,  Stroma  of  the  gland. — {From  Nagel,  after  Basch.) 

Although  the  mammary  glands  are  typically  functional  only  in 
females  in  the  period  immediately  succeeding  pregnancy,  cases  are  not 
unknown  in  which  the  glands  have  been  well  developed  and  functional  in 
males  {gynecomastia).  Furthermore,  a  functional  activity  of  the  glands 
normally  occurs  immediately  after  birth,  infants  of  both  sexes  yielding  a 
few  drops  of  a  milky  fluid,  the  so-called  witch-milk  (Hexenmilch) ,  when 
the  glands  are  subjected  to  pressure. 

LITERATURE. 

J.  T.  Bowen:  "The  Epitrichial  Layer  of  the  Human  Epidermis,"  Anat.  Anzeiger,  rv 

1889. 
Brouha:  •' Recherches  sur  les  diverses  phases  du  developpement  et  de  l'activite  dela 

mammelle,"  Arch,  de  Biol.,  xxi,  1905. 
G.  Burckhard:  "Ueber  embryonale  Hypermastie  und  Hyperthelie,"  Anat.  Hefte 

viii,  1897. 
H.  Head:  "On  Disturbances  of  Sensation  with  Special  Reference  to  the  Pain  of 

Visceral  Disease,"  Brain,  xvi,  1892;  xvn,  1894;  and  xix,  1896. 
E.   Kallius:  "Ein  Fall  von  Milchleiste  bei  einem  menschlichen  Embryo,"   Anat. 

Hefte,  viii,  1897. 


152  LITERATURE 

T.   Okamura:  "Ueber  die  Entwicklung  des  Nagels  beim  Menschen,"   Archiv  fur 

Dermatol,  und  Syphilol.,  xxv,  1900. 
H.  Schmidt:  "Ueber  normale  Hyperthelie  menschlicher  Embryonen  und  uber  die 

erste  Anlage  der  menschlichen  Milchdriisen  iiberhaupt,"  Morphol.  Arbeiten,  xvil, 

1897. 
C.  S.  Sherrington:  "Experiments  in  Examination  of  the  Peripheral  Distribution  of 

the  Fibres  of  the  Posterior  Roots  of  some  Spinal  Nerves,"  Philos.  Trans.  Royal 

Soc,  clxxxiv,  1893,  and  cxc,  1898. 
P.   Stohr:  " Entwickelungsgeschichte  des  menschlichen  Wollhaares,"   Anat.  Hefte, 

xxiii,  1903. 
H.  Strahl:  "Die  erste  Entwicklung  der  Mammarorgane  beim  Menschen,"  Verhandl. 

Anat.  Gesellsch.,  xii,  1898. 


CHAPTER  VII. 

THE    DEVELOPMENT    OF    THE    CONNECTIVE    TISSUES 
AND  SKELETON. 

It  has  been  seen  that  the  cells  of  a  very  considerable  portion  of 
the  somatic  and  splanchnic  mesoderm,  as  well  as  of  parts  of  the 
mesodermic  somites,  become  converted  into  mesenchyme.  A 
very  considerable  portion  of  this  becomes  converted  into  what  are 
termed  connective  or  supporting  tissues,  characterized  by  consisting 
of  a  non-cellular  matrix  in  which  more  or  less  scattered  cells  are 
embedded.  These  tissues  enter  to  a  greater  or  less  extent  into  the 
formation  of  all  the  organs  of  the  body,  with  the  exception  of  those 
forming  the  central  nervous  system,  and  constitute  a  network  which 
holds  together  and  supports  the  elements  of  which  the  organs  are 
composed;  in  addition,  they  take  the  form  of  definite  membranes 
(serous  membranes,  fasciae),  cords  (tendons,  ligaments),  or  solid 
masses  (cartilage),  or  form  looser  masses  or  layers  of  a  somewhat 
spongy  texture  (areolar  tissue).  The  intermediate  substance  is 
somewhat  varied  in  character,  being  composed  sometimes  of  white, 
non-branching,  non-elastic  fibers,  sometimes  of  yellow,  branching, 
elastic  fibers,  of  white,  branching,  but  inelastic  fibers  which  form 
a  reticulum,  or  of  a  soft  gelatinous  substance  containing  considerable 
quantities  of  mucin,  as  in  the  tissue  which  constitutes  the  Whartonian 
jelly  of  the  umbilical  cord.  Again,  in  cartilage  the  matrix  is  com- 
pact and  homogeneous,  or,  in  other  cases,  more  or  less  fibrous, 
passing  over  into  ordinary  fibrous  tissue,  and,  finally,  in  bone  the 
organic  matrix  is  largely  impregnated  with  salts  of  lime. 

Two  views  exist  as  to  the  mode  of  formation  of  the  matrix,  some 
authors  maintaining  that  in  the  fibrous  tissues  it  is  produced  by  the 
actual  transformation  of  the  mesenchyme  cells  into  fibers,  while 
others  claim  that  it  is  manufactured  by  the  cells  but  does  not  directly 

i53 


154  DEVELOPMENT    OF    CONNECTIVE    TISSUE 

represent  the  cells  themselves.  Fibrils  and  material  out  of  which 
fibrils  could  be  formed  have  undoubtedly  been  observed  in  connec- 
tive-tissue cells,  but  whether  or  not  these  are  later  passed  to  the 
exterior  of  the  cell  to  form  a  connective-tissue  fiber  is  not  yet  certain, 
and  on  this  hangs  mainly  the  difference  between  the  theories. 
Recently  it  has  been  held  (Mall)  that  the  mesenchyme  of  the  embryo 
is  really  a  syncytium  in  and  from  the  protoplasm  of  which  the  matrix 


W/^ii 


\%jfL 


m^mmF% 


■a  fmm0m^& 


■J 


> 


Fig.  91. — Portion  of  the  Center  of  Ossification  of  the  Parietal  Bone  of  a 

Human  Embryo. 


forms;  if  this  be  correct,  the  distinction  which  the  older  views  make 
between  the  intercellular  and  intracellular  origin  of  the  matrix 
becomes  of  little  importance. 

Bone  differs  from  the  other  varieties  of  connective  tissue  in  that 
it  is  never  a  primary  formation,  but  is  always  developed  either  in 
fibrous  tissue  or  cartilage;  and  according  as  it  is  associated  with  the 
one  or  the  other,  it  is  spoken  of  as  membrane  bone  or  cartilage  bone. 
In  the  development  of  membrane  bone  some  of  the  connective-tissue 
cells,  which  in  consequence  become  known  as  osteoblasts,  deposit 
lime  salts  in  the  matrix  in  the  form  of  bony  spicules  which  increase 
in  size  and  soon  unite  to  form  a  network  (Fig.  91).  The  trabecular 
of  the  network  continue  to  thicken,  while,  at  the  same  time,  the  forma- 
tion of  spicules  extends  further  out  into  the  connective-tissue  mem- 
brane, radiating  in  all  directions  from  the  region  in  which  it  first 


DEVELOPMENT    OF    BONE 


155 


developed.     Later  the  connective  tissue  which  lies  upon  either  sur- 
face of  the  reticular  plate  of  bone  thus  produced  condenses  to  form 
a  stout  membrane,  the  periosteum,  between  which  and  the  osseous 
plate  osteoblasts  arrange  themselves  in  a  more  or  less  definite  layer 
and  deposit  upon  the  surface  of  the  plate  a  lamella  of  compact  bone. 
A  membrane  bone,  such  as  one  of  the  flat  bones  of  the  skull,  thus 
comes  to  be  composed  of  two  plates 
of  compact  bone,  the  inner  and 
outer  tables,  enclosing  and  united 
to  a  middle  plate  of  spongy  bone 
which  constitutes  the  diploe. 

With  bones  formed  from  carti- 
lage the  process  is  somewhat  dif- 
ferent. In  the  center  of  the 
cartilage  the  intercellular  matrix 
becomes  increased  so  that  the  cells 
appear  to  be  more  scattered  and 
a  calcareous  deposit  forms  in  it. 
All  around  this  region  of  calcifica- 
tion the  cells  arrange  themselves 
in  rows  (Fig.  92)  and  the  process 
of  calcification  extends  into  the 
trabecular  of  matrix  which  separate 
these  rows.  While  these  processes 
have  been  taking  place  the  mesen- 
chyme surrounding  the  cartilage 
has  become  converted  into  a 
periosteum  (po),  similar  to  that  of  membrane  bone,  and  its  osteo- 
blasts deposit  a  layer  of  bone  (p)  upon  the  surface  of  the  cartilage. 
The  cartilage  cells  now  disappear  from  the  intervals  between  the 
trabeculae  of  calcified  matrix,  which  form  a  fine  network  into  which 
masses  of  mesenchyme  (Fig.  93,  pi),  containing  blood-vessels  and 
osteoblasts,  here  and  there  penetrate  from  the  periosteum,  after 
having  broken  through  the  layer  of  periosteal  bone.  These  masses 
absorb  a  portion  of  the  fine  calcified  network  and  so  transform  it 


po 


P 


Fig  92. — Longitudinal  section  of 
Phalanx  of  a  Finger  of  an  Embryo 
of  3  1/2  Months. 

c,  Cartilage  trabeculae;  p,  periosteal 
bone;  po,  periosteum;  x,  ossification 
center. — (Szymonowicz.) 


i56 


DEVELOPMENT    OF    BONE 


po 


pi 


C      ^; 


into  a  coarse  network,  the  meshes  of  which  they  occupy  to  form 
the  bone  maigow  (m),  and  the  osteoblasts  which  they  contain  arrange 
themselves  on  the  surface  of  the  persisting  trabeculse  and  deposit 
layers  of  bone  upon  their  surfaces.  In  the  meantime  the  calcifica- 
tion of  the  cartilage  matrix  has  been  extending,  and  as  fast  as  the 

network  of  calcified  trabeculse  is 
formed  it  is  invaded  by  the  mesen- 
chyme, until  finally  the  cartilage 
becomes  entirely  converted  into  a 
mass  of  spongy  bone  enclosed 
within  a  layer  of  more  compact 
periosteal  bone. 

As  a  rule,  each  cartilage  bone 
is  developed  from  a  single  center 
of  ossification,  and  when  it  is  found 
that  a  bone  of  the  skull,  for  in- 
stance, develops  by  several  cen- 
ters, it  is  to  be  regarded  as  formed 
by  the  fusion  of  several  primarily 
distinct  bones,  a  conclusion  which 
may  generally  be  confirmed  by  a 
comparison  of  the  bone  in  ques- 
tion with  its  homologues  in  the 
lower  vertebrates.  Exceptions  to 
this  rule  occur  in  bones  situated  in  the  median  line  of  the  body, 
these  occasionally  developing  from  two  centers  lying  one  on  either 
side  of  the  median  line,  but  such  centers  are  usually  to  be  regarded 
as  a  double  center  rather  than  as  two  distinct  centers,  and  are 
merely  an  expression  of  the  fundamental  bilaterality  which  exists 
even  in  median  structures. 

More  striking  exceptions  are  to  be  found  in  the  long  bones  in 
which  one  or  both  extremities  develop  from  special  centers  which 
give  rise  to  the  epiphyses  (Fig.  94,  ep,  ep'),  the  shaft  or  diaphysis  (d) 
being  formed  from  the  primary  center.  Similar  secondary  centers 
appear  in  marked  prominences  on  bones  to  which  powerful  muscles 


Fig.  93. — The  Ossification  Center 
of  Fig.  92  More  Highly  Magnified. 
c,  Ossifying  trabeculse;  cc,  cavity  of 
cartilage  network;  m,  marrow  cells;  p, 
periosteal  bone;  pi,  irruption  of  peri- 
osteal tissue;  po,  periosteum. — (Szymo- 
nowicz.) 


DEVELOPMENT    OF   BONE 


157 


are  attached  (Fig.  94,  a  and  b),  but  these,  as  well  as  the  epiphysial 
centers,  can  readily  be  recognized  as  secondary  from  the  fact  that 
they  do  not  appear  until  much  later  than  the  primary  centers  of  the 
bones  to  which  they  belong.  These  secondary 
centers  give  the  necessary  firmness  required 
for  articular  surfaces  and  for  the  attachment 
of  muscles  and,  at  the  same  time,  make  pro- 
vision for  the  growth  in  length  of  the  bone, 
since  a  plate  of  cartilage  always  intervenes 
between  the  epiphyses  and  the  diaphysis. 
This  cartilage  continues  to  be  transformed 
into  bone  on  both  its  surfaces  by  the  extension 
of  both  the  epiphysial  and  diaphysial  ossifica- 
tion into  it,  and,  at  the  same  time,  it  grows 
in  thickness  with  equal  rapidity  until  the 
bone  reaches  its  required  length,  whereupon 
the  rapidity  of  the  growth  of  the  cartilage 
diminishes  and  it  gradually  becomes  com- 
pletely ossified,  uniting  together  the  epiphysis 
and  diaphysis. 

The  growth  in  thickness  of  the  long  bones 
is,  however,  an  entirely  different  process,  and 
is  due  to  the  formation  of  new  layers  of  peri- 
osteal bone  on  the  outside  of  those  already 
present.  But  in  connection  with  this  process' 
an  absorption  of  bone  also  takes  place.  A 
section  through  the  middle  of  the  shaft  of  a 
humerus,  for  example,  at  an  early  stage  of 
development  would  show  a  peripheral  zone  of 
compact  bone  surrounding  a  core  of  spongy  bone,  the  meshes  of  the 
latter  being  occupied  by  the  marrow  tissue.  A  similar  section  of 
an  adult  bone,  on  the  other  hand,  would  show  only  the  peripheral 
compact  bone,  much  thicker  than  before  and  enclosing  a  large 
marrow  cavity  in  which  no  trace  of  spongy  bone  might  remain. 
The  difference  depends  on  the  fact  that  as  the  periosteal  bone 


Fig.  94. — The  Ossi- 
fication Centers  of 
the  Femur. 

a,  and  b,  Secondary 
centers  for  the  great  and 
lesser  trochanters;  d, 
diaphysis;  ep,  upper  and 
ep',  lower  epiphysis. — 
(Testut.) 


158  DEVELOPMENT    OF    BONE 

increases  in  thickness,  there  is  a  gradual  absorption  of  the  spongy 
bone  and  also  of  the  earlier  layers  of  periosteal  bone,  this  absorption 
being  carried  on  by  large  multinucleated  cells,  termed  osteoclasts, 
derived  from  the  marrow  mesenchyme.  By  their  action  the  bone 
is  enabled  to  reach  its  requisite  diameter  and  strength,  without 
becoming  an  almost  solid  and  unwieldy  mass  of  compact  bone. 

During  the  ossification  of  the  cartilaginous  trabeculse  osteoblasts 
become  enclosed  by  the  bony  substance,  the  cavities  in  which  they 
lie  forming  the  lacuna  and  processes  radiating  out  from  them,  the 


Fig.   95. — A,  Transverse  Section  of  the  Femur  of  a  Pig  Killed  after 
Having  Been  fed  with  Madder  for  Four  Weeks;  B,  the  Same  of  a  Pig  Killed 
Two  Months  after  the  Cessation  of  the  Madder  Feeding. 
The  heavy  black  line  represents  the  portion  of  bone  stained  by  the  madder. — (After 

Flourens.) 

canaliculi,  so  characteristic  of  bone  tissue.  In  the  growth  of  peri- 
osteal bone  not  only  do  osteoblasts  become  enclosed,  but  blood- 
vessels also,  the  Haversian  canals  being  formed  in  this  way,  and 
around  these  lamellae  of  bone  are  deposited  by  the  enclosed  osteo- 
blasts to  form  Haversian  systems. 

That  the  absorption  of  periosteal  bone  takes  place  during  growth 
can  be  demonstrated  by  taking  advantage  of  the  fact  that  the  coloring 
substance  madder,  when  consumed  with  food,  tinges  the  bone  being 
formed  at  the  time  a  distinct  red.  In  pigs  fed  with  madder  for  a  time 
and  then  killed  a  section  of  the  femur  shows  a  superficial  band  of  red  bone 
(Fig.  95,  A),  but  if  the  animals  be  allowed  to  live  for  one  or  two  months 
after  the  cessation  of  the  madder  feeding,  the  red  band  will  be  found  to  be 
covered  by  a  layer  of  white  bone  varying  in  thickness  according  to  the 
interval  elapsed  since  the  cessation  of  feeding  (Fig.  95,  B);  and  if  this 


DEVELOPMENT    OF    THE    SKELETON 


159 


interval  amount  to  four  months,  it  will  be  found  that  the  thickness  of  the 
uncolored  bone  between  the  red  bone  and  the  marrow  cavity  will  have 
greatly  diminished  (Flourens). 

The  Development  of  the  Skeleton. — Embryologically  con- 
sidered, the  skeleton  is  composed  of  two  portions,  the  axial  skeleton, 
consisting  of  the  skull,  the  vertebrae,  ribs,  and  sternum,  developing 


fin*  '  \m.: 


SCcr 


tsa 


ri> 


<*r 


Fig.  96. — Frontal  Section  through  Mesodermic  Somites  of  a  Calf  Embryo. 

isa,  Intersegmental  artery;  my,  myotome;  n,  central  nervous  system;  nc,  notochord; 

sea  and  scp,  anterior  and  posterior  portions  of  sclerotomes. 

from  the  sclerotomes  of  the  mesodermal  somites,  and  the  appen- 
dicular skeleton,  which  includes  the  pectoral  and  pelvic  girdles  and 
the  bones  of  the  limbs,  and  which  arises  from  the  mesenchyme  of 
the  somatic  mesoderm.  It  will  be  convenient  to  consider  first  the 
development  of  the  axial  skeleton,  and  of  this  the  differentiation  of 
the  vertebral  column  and  ribs  may  first  be  discussed. 


i6o 


DEVELOPMENT    OP    THE    VERTEBRAE 


The  Development  of  the  Vertebrae  and  Ribs. — The  mesen- 
chyme formed  from  the  sclerotome  of  each  mesodermic  somite 
grows  inward  toward  the  median  line  and  forms  a  mass  lying 
between  the  notochord  and  the  myotome,  separated  from  the 
similar  mass  in  front  and  behind  by  some  loose  tissue  in  which  lies 
an  intersegmental  artery.     Toward  the  end  of  the  third  week  of 

development  the  cells  of  the 
posterior  portion  of  each  sclero- 
tome condense  to  a  tissue  more 
compact  than  that  of  the  anterior 
portion  (Fig.  96),  and  a  little 
later  the  two  portions  become 
separated  by  a  cleft.  At  about 
the  same  time  the  posterior  por- 
tion sends  a  process  medially,  to 
enclose  the  notochord  by  uniting 
with  a  corresponding  process 
from  the  sclerotome  of  the  other 
side,  and  it  also  sends  a  pro- 
longation dorsally  between  the 
myotome  and  the  spinal  cord  to 
form  the  vertebral  arch,  and  a 
third  process  laterally  and  ven- 
trally  along  the  distal  border  of 
the  myotome  to  form  a  costal  process  (Fig.  97).  The  looser  tissue 
of  the  anterior  half  of  the  sclerotome  also  grows  medially  to  sur- 
round the  notochord,  filling  up  the  intervals  between  successive 
denser  portions,  and  it  forms  too  a  membrane  extending  between 
successive  vertebral  arches.  Later  the  tissue  surrounding  the  noto- 
chord, which  is  derived  from  the  anterior  half  of  the  sclerotome, 
associates  itself  with  the  posterior  portion  of  the  preceding  sclerotome 
to  form  what  will  later  be  a  vertebra,  the  tissue  occupying  and 
adjacent  to  the  line  of  division  between  the  anterior  and  posterior 
portions  of  the  sclerotomes  condensing  to  form  intervertebral 
fibrocartilages.     Consequently   each   vertebra  is  formed  by  parts 


Fig.  97. — Transverse  Section 
through  the  intervertebral  plate 
of  the  First  Cervical  Vertebra  of  a 
Calf  Embryo  of  8.8  mm. 

be1,  Intervertebral  plate;  mi,  fourth 
myotome;  s,  hypochordal  bar;  XI,  spinal 
accessory  nerve. — (Froriep.) 


DEVELOPMENT  OF  THE  VERTEBRA  l6l 

from  two  sclerotomes,  the  original  intersegmental  artery  passes  over 
the  body  of  a  vertebra,  and  the  vertebrae  themselves  alternate  with 
the  myotomes.  With  this  differentiation  the  first  or  blastemic  stage 
of  the  development  of  the  vertebras  closes. 

In  the  second  or  cartilaginous  stage,  portions  of  the  sclerotomic 
mesenchyme  become  transformed  into  cartilage.  In  the  posterior 
portion  of  each  vertebral  body,  that  is  to  say  in  the  portion  formed 
from  the  anterior  halves  of  the  more  posterior  of  the  two  pairs  of 
sclerotomes  entering  into  its  formation,  two  centers  of  chondrifica- 
tion  appear,  one  on  each  side  of  the  median  line,  and  these  eventually 
unite  to  form  a  single  cartilaginous  body,  the  chondrification  prob- 
ably also  extending  to  some  extent  into  the  denser  anterior  portion 
of  the  body.  A  center  also  appears  in  each  half  of  the  vertebral 
arch  and  in  each  costal  process,  the  cartilages  formed  in  the  costal 
processes  of  the  anterior  cervical  region  uniting  across  the  median 
line  below  the  notochord,  to  form  what  has  been  termed  a  hypo- 
chordal  bar  (Figs.  97  and  98).  These  bars  are  for  the  most  part 
but  transitory,  recalling  structures  occurring  in  the  lower  vertebrates; 
in  the  mammalia  they  degenerate  before  the  close  of  the  cartilaginous 
stage  of  development,  except  in  the  case  of  the  atlas,  whose  develop- 
ment will  be  described  later.  As  development  proceeds  the  cartil- 
ages of  the  vertebral  arches  and  costal  processes  increase  in  length 
and  come  into  contact  with  the  cartilaginous  bodies,  with  which 
they  eventually  fuse,  and  from  the  vertebral  arches  processes  grow 
out  which  represent  the  future  transverse  and  articular  processes. 

The  fusion  of  the  cartilage  of  the  costal  process  with  the  body  of 
the  vertebra  does  not,  however,  persist.  Later  a  solution  of  the 
junction  occurs  and  the  process  becomes  a  rib  cartilage,  the  mesen- 
chyme surrounding  the  area  of  solution  forming  the  costo-vertebral 
ligaments.  At  first  the  rib  cartilage  is  separated  by  a  distinct 
interval  from  the  transverse  process  of  the  vertebral  arch,  but  later 
it  develops  a  process,  the  tubercle,  which  bridges  the  gap  and  forms 
an  articulation  with  the  transverse  process. 

The  mesenchyme  which  extends  between  successive  vertebral 
arches  does  not  chondrify,  but  later  becomes  transformed  into  the 


162  DEVELOPMENT    OF    THE   VERTEBRAE 

interspinous  ligaments  and  the  ligamenta  ftava,  while  the  anterior 
and  posterior  longitudinal  ligaments  are  formed  from  unchondrified 
portions  of  the  tissue  surrounding  the  vertebral  bodies. 

As  was  pointed  out,  the  mesenchyme  in  the  region  of  the  cleft 
separating  the  anterior  and  posterior  portions  of  a  sclerotome  be- 
comes an  intervertebral  fibrocartilage,  and,  as  the  cartilaginous 
bodies  develop,  the  portions  of  the  notochord  enclosed  by  them 
become  constricted,  while  at  the  same  time  the  portions  in  the 
intervertebral  regions  increase  in  size.  Finally  the  notochord  dis- 
appears from  the  vertebral  regions,  although  a  canal,  representing 
its  former  position,  traverses  each  body  for  a  considerable  time,  but 
in  the  intervertebral  regions  it  persists  as  relatively  large  flat  disks 
forming  the  pulpy  nuclei  of  the  fibrocartilages. 

The  mode  of  development  described  above  applies  to  the  great 
majority  of  the  vertebrae,  but  some  departures  from  it  occur,  and 
these  may  be  conveniently  considered  before  passing  on  to  an 
account  of  the  ossification  of  the  cartilages.  The  variations  affect 
principally  the  extremes  of  the  series.  Thus  the  posterior  vertebrae 
present  a  reduction  of  the  vertebral  arches,  those  of  the  last  sacral 
vertebrae  being  but  feebly  developed,  while  in  the  coccygeal  vertebrae 
they  are  indicated  only  in  the  first.  In  the  first  cervical  vertebra, 
the  atlas,  the  reverse  is  the  case,  for  the  entire  adult  vertebra  is 
formed  from  the  posterior  portion  of  a  sclerotome,  its  lateral  masses 
and  posterior  arch  being  the  vertebral  arches,  while  its  anterior  arch 
is  the  hypochordal  bar,  which  persists  in  this  vertebra  only.  A  well- 
developed  centrum  is  also  formed,  however  (Fig.  98),  but  it  does  not 
unite  with  the  parts  derived  from  the  preceding  sclerotome,  but 
during  its  ossification  unites  with  the  centrum  of  the  epistropheus 
(axis),  forming  the  odontoid  process  of  that  vertebra.  The  epistro- 
pheus consequently  is  formed  by  one  and  a  half  sclerotomes,  while 
but  half  a  one  constitutes  the  atlas. 

The  extent  to  which  the  ribs  are  developed  in  connection  with 
the  various  vertebrae  also  varies  considerably.  Throughout  the  cer- 
vical region  they  are  short,  the  upper  five  or  six  being  no  longer  than 
the  transverse  processes,  with  the  tips  of  which  their  extremities 


DEVELOPMENT  OF  THE  VERTEBRA 


163 


unite  at  an  early  stage.  In  the  upper  five  or  six  vertebrae  a  relatively 
large  interval  persists  between  the  rib  and  the  transverse  process, 
forming  the  costo-transverse  foramen,  through  which  the  vertebral 
vessels  pass,  but  in  the  seventh  vertebra  the  fusion  is  more  extensive 
and  the  foramen  is  very  small  and  hardly  noticeable.  In  the  thoracic 
region  the  ribs  reach  their  greatest  development,  the  upper  eight  or 


■  \;-^Z ...  --■ 


Fig.   9,8. — Longitudinal  Section  through  the   Occipital  Region  and  Upper 

Cervical  Vertebrae  of  a  Calf  Embryo  of  18.5  mm. 

has,    Basilar    artery;    ch,    notochord;    Kcl~ 4,   vertebral   centra;    lc2~4,  intervertebral 

disks;  occ,  basioccipital;  Scx~4,  hypochordal  bars. — (Froriep.) 


nine  extending  almost  to  the  mid-ventral  line,  where  their  extremities 
unite  to  form  a  longitudinal  cartilaginous  bar  from  which  the  sternum 
develops  (see  p.  166).  The  lower  three  or  four  thoracic  ribs  are 
successively  shorter,  however,  and  lead  to  the  condition  found  in 
the  lumbar  vertebras,  where  they  are  again  greatly  reduced  and 
firmly  united  with  the  transverse  processes,  the  union  being  so  close 
that  only  the  tips  of  the  latter  can  be  distinguished,  forming  what 
are  known  as  the  accessory  tubercles.     In  the  sacral  region  the  ribs 


164 


DEVELOPMENT  OF  THE  VERTEBRA 


are  reduced  to  short  flat  plates,  which  unite  together  to  form  the 
lateral  masses  of  the  sacrum,  and,  finally,  in  the  coccygeal  region  the 
blastemic  costal  processes  of  the  first  vertebra  unite  with  the  trans- 
verse processes  to  form  the  transverse  processes  of  the  adult  vertebra, 
but  no  indications  of  them  are  to  be  found  in  the  other  vertebrae 
beyond  the  blastemic  stage. 

The  third  stage  in  the  development  of  the  axial  skeleton  begins 
with  the  ossification  of  the  cartilages,  and  in  each  vertebra  there  are 
typically  as  many  primary  centers  of  ossification  as  there  were 
originally  cartilages,  except  that  but  a  single  center  appears  in  the 
body.  Thus,  to  take  a  thoracic  vertebra  as  a  type,  a  center  appears 
in  each  half  of  each  vertebral  arch  at  the  base  of  the  transverse  process 
and  gradually  extends  to  form  the  bony  lamina,  pedicle,  and  the 
greater  portion  of  the  transverse  and  spinous  processes;  a  single 
center  gives  rise  to  the  body  of  the  vertebra;  and  each  rib  ossifies 


Fig.  99. — A,  A  Vertebra  at  Birth;  B,  Lumbar  Vertebra  showing  Secondary 
Centers  of  Ossification. 
a,  Center  for  the  articular  process;  c,  body;  el,  lower  epiphysial  plate;  en,  upper 
epiphysial  plate;  na,  vertebral  arch;  s,  center  for  spinous  process;  t,  center  for  transverse 
process. — (Sappey.) 


from  a  single  center.  These  various  centers  appear  early  in  embry- 
onic life,  but  the  complete  transformation  of  the  cartilages  into  bone 
does  not  occur  until  some  time  after  birth,  each  vertebra  at  that 
period  consisting  of  three  parts,  a  body  and  two  halves  of  an  arch, 
separated  by  unossified  cartilage  (Fig.  99,  A).  At  about  puberty 
secondary  centers  make  their  appearance;  one  appears  in  the  carti- 
lage which  still  covers  the  anterior  and  posterior  surfaces  of  the 
vertebral  body,  producing  disks  of  bone  in  these  situations  (Fig.  99, 


DEVELOPMENT  OF  THE  VERTEBRA 


l65 


B,  en  and  el),  another  appears  at  the  tip  of  each  spinous  and  trans- 
verse process  (Fig.  99,  B),  and  in  the  lumbar  vertebrae  others  appear 
at  the  tips  of  the  articulating  processes.  The  epiphyses  so  formed 
remain  separate  until  growth  is  completed  and  between  the  sixteenth 
and  twenty-fifth  years  unite  with  the  bones  formed  from  the  primary 
centers,  which  have  fused  by  this  time,  to  form  a  single  vertebra. 

Each  rib  ossifies  from  a  single  primary  center  situated  near  the 
angle,  secondary  centers  appearing  for  the  capitulum  and  tuberosity. 

In  some  of  the  vertebras  modifications  of  this  typical  mode  of 
ossification  occur.  Thus,  in  the  upper  five  cervical  vertebrae  the 
centers  for  the  rudimentary  ribs  are  suppressed,  ossification  extend- 
ing into  them  from  the  vertebral  arch  centers,  and  a  similar  suppres- 
sion of  the  costal  centers  occurs  in  the  lower  lumbar  vertebrae,  the 
first  only  developing  a  separate  rib  center.     Furthermore,  in  the 


Fig.  ioo.—A,  Upper  Surface  of  the  First  Sacral  Vertebra,  and  B,  Ventral 

View  of  the  Sacrum  showing  Primary  Centers  of  Ossification. 

c,  Body;  na,  vertebral  arch;  r,  rib  center. — (Sappey.) 

atlas  a  double  center  appears  in  the  persisting  hypochordal  bar,  and 
the  body  which  corresponds  to  the  atlas,  after  developing  the  termi- 
nal epiphysial  disks,  fuses  with  the  body  of  the  epistropheus  (axis) 
to  form  its  odontoid  process,  this  vertebra  consequently  possessing, 
in  addition  to  the  typical  centers,  one  (double)  other  primary  and  two 
secondary  centers.     In  the  sacral  region  the  typical  centers  appear 


1 66  DEVELOPMENT    OF    THE    STERNUM 

in  all  five  vertebrae,  with  the  exception  of  rib  centers  for  the  last  one 
or  two  (Fig.  ioo)  and  two  additional  secondary  centers  give  rise  to 
plate-like  epiphyses  on  each  side,  the  upper  plates  forming  the 
articular  surface  for  the  ilium.  At  about  the  twenty-fifth  year  all 
the  sacral  vertebrae  unite  to  form  a  single  bone,  and  a  similar  fusion 
occurs  also  in  the  rudimentary  vertebrae  of  the  coccyx. 

The  majority  of  the  anomalies  seen  in  the  vertebral  column  are  due 
to  the  imperfect  development  of  one  or  more  cartilages  or  of  the  centers  of 
ossification.  Thus,  a  failure  of  an  arch  to  unite  with  the  body  or  even  the 
complete  absence  of  an  arch  or  half  an  arch  may  occur,  and  in  cases  of 
spina  bifida  the  two  halves  of  the  arches  fail  to  unite  dorsally.  Occasion- 
ally the  two  parts  of  the  double  cartilaginous  center  for  the  body  fail  to 
unite,  a  double  body  resulting;  or  one  of  the  two  parts  may  entirely  fail, 
the  result  being  the  formation  of  only  one-half  of  the  body  of  the  vertebra. 
Other  anomalies  result  from  the  excessive  development  of  parts.  Thus, 
the  rib  of  the  seventh  cervical  vertebra  may  sometimes  remain  distinct  and 
be  long  enough  to  reach  the  sternum,  and  the  first  lumbar  rib  may  also 
fail  to  unite  with  Its  vertebra.  On  the  other  hand,  the  first  thoracic  rib  is 
occasionally  found  to  be  imperfect. 

The  Development  of  the  Sternum. — Longitudinal  bars,  which 
are  formed  by  the  fusion  of  the  ventral  ends  of  the  anterior  eight  or 
nine  cartilaginous  thoracic  ribs,  represent  the  future  sternum.  At 
an  early  period  the  two  bars  come  into  contact  anteriorly  and  fuse 
together  (Fig.  101),  and  at  this  anterior  end  two  usually  indistinctly 
separated  masses  of  cartilage  are  to  be  observed  at  the  vicinity  of  the 
points  where  the  ventral  ends  of  the  cartilaginous  clavicles  articulate. 
These  are  the  episternal  cartilages  {em),  which  later  normally  unite 
with  the  longitudinal  bars  and  form  part  of  the  manubrium  sterni, 
though  occasionally  they  persist  and  ossify  to  form  the  ossa  supraster- 
nal. The  fusion  of  the  longitudinal  bars  gradually  extends  back- 
ward until  a  single  elongated  plate  of  cartilage  results,  with  which  the 
seven  anterior  ribs  are  united,  one  or  two  of  the  more  posterior  ribs 
which  originally  took  part  in  the  formation  of  each  bar  having 
separated.  The  portions  of  the  bars  formed  by  these  posterior  ribs 
constitute  the  xiphoid  process. 

The  ossification  of  the  sternum  (Fig.  102)  partakes  to  a  certain 
extent  of  the  original  bilateral  segmental  origin  of  the  cartilage, 


DEVELOPMENT    OF    THE    STERNUM  167 

but  there  is  a  marked  condensation  of  the  centers  of  ossification  and 
considerable  variation  in  their  number  also  occurs.  In  the  portion  of 
the  cartilage  which  lies  below  the  junction  of  the  third  costal  cartilages 
a  series  of  pairs  of  centers  appears  just  about  birth,  each  center 


Fig.  ioi. — Formation  of  the  Sternum  in  an  Embryo  of  About  3  cm. 
el,  Clavicle;  em,  episternal  cartilage. — (Ruge.) 


probably  representing  an  epiphysial  center  of  a  corresponding  rib. 
Later  the  centers  of  each  pair  fuse  and  the  single  centers  so  formed, 
extending  through  the  cartilage,  eventually  unite  to  form  the  greater 
part  of  the  body  of  the  bone.  In  each  of  the  two  uppermost  seg- 
ments, however,  but  a  single  center  appears,  that  of  the  second 
segment  uniting  with  the  more  posterior  centers  and  forming  the 
upper  part  of  the  body,  while  the  uppermost  center  gives  rise  to  the 
manubrium,  which  frequently  persists  as  a  distinct  bone  united  to  the 
body  by  a  hinge-joint. 


i68 


DEVELOPMENT    OF    THE    SKULL 


A  failure  of  the  cartilaginous  bars  to  fuse  produces  the  condition 
known  as  cleft  sternum,  or  if  the  failure  to  fuse  affects  only  a  portion  of  the 
bars  there  results  a  perforated  sternum.  A  perforation  or  notching  of  the 
xiphoid  cartilage  is  of  frequent  occurrence  owing  to  this  being  the  region 
where  the  fusion  of  the  bars  takes  place  last. 


Fig.     i  02. — Sternum     of 
New-born    Child,    showing 
Centers  of  Ossification. 
I  to  VII,  Costal  cartilages. — 
(Gegenbaur.) 


Fig.  103. — Reconstruction  of  the  Chondro- 
cranium  of  an  embryo  of  14  mm. 
as,  Alisphenoid;  bo,  basioccipital;  bs,  basi- 
sphenoid;  eo,  exoccipital;  m,  Meckel's  cartilage; 
os,  orbitosphenoid;  p,  periotic;  ps,  presphenoid; 
so,  sella  turcica;  s,  supraoccipital. — {Levi.) 


The  suprasternal  bones  are  the  rudiments  of  a  bone  or  cartilage,  the 
omosternum,  situated  in  front  of  the  manubrium  in  many  of  the  lower 
mammalia.  It  furnishes  the  articular  surfaces  for  the  clavicles  and  is 
possibly  formed  by  a  fusion  of  the  ventral  ends  of  the  cartilages  which 
represent  those  bones;  hence  its  appearance  as  a  pair  of  bones  in  the  rudi- 
mentary condition. 

The  Development  of  the  Skull. — In  its  earliest  stages  the 
human  skull  is  represented  by  a  continuous  mass  of  mesenchyme 
which  invests  the  anterior  portion  of  the  notochord  and  extends 
forward  beyond  its  extremity  into  the  nasal  region,  forming  a  core 
for  the  nasal  process  (see  p.  99).  From  each  side  of  this  basal  mass 
a  wing  projects  dorsally  to  enclose  the  anterior  portion  of  the  medul- 
lary canal  which  will  later  become  the  cerebral  part  of  the  central 
nervous  system.     No  indications  of  a  segmental  origin  are  to  be 


DEVELOPMENT    OF    THE    SKULL  169 

found  in  this  mesenchyme;  as  stated,  it  is  a  continuous  mass,  and 
this  is  likewise  true  of  the  cartilage  which  later  develops  in  it. 

The  chondrihcation  occurs  first  along  the  median  line  in  what 
will  be  the  occipital  and  sphenoidal  regions  of  the  skull  (Fig.  103) 
and  thence  gradually  extends  forward  into  the  ethmoidal  region  and 
to  a  certain  extent  dorsally  at  the  sides  and  behind  into  the  regions 
later  occupied  by  the  wings  of  the  sphenoid  (as  and  os)  and  the 
squamous  portion  of  the  occipital  (s).  No  cartilage  develops, 
however,  in  the  rest  of  the  sides  or  in  the  roof  of  the  skull,  but  the 
mesenchyme  of  these  regions  becomes  converted  into  a  dense  mem- 
brane of  connective  tissue.  While  the  chondrification  is  proceeding 
in  the  regions  mentioned,  the  mesenchyme  which  encloses  the 
internal  ear  becomes  converted  into  cartilage,  forming  a  mass,  the 
periotic  capsule  (Fig.  103,  p),  wedged  in  on  either  side  between  the 
occipital  and  sphenoidal  regions,  with  which  it  eventually  unites  to 
form  a  continuous  chondro cranium,  perforated  by  foramina  for  the 
passage  of  nerves  and  vessels. 

The  posterior  part  of  the  basilar  portion  of  the  occipital  cartilage 
presents  certain  peculiarities  of  development.  In  calf  embryos 
there  are  in  this  region,  in  very  early  stages,  four  separate  condensa- 
tions of  mesoderm  corresponding  to  as  many  mesodermic  somites 
and  to  the  three  roots  of  the  hypoglossal  nerve  together  with  the  first 
cervical  or  suboccipital  nerve  (Froriep)  (Fig.  104).  These  mesenchy- 
mal masses  in  their  general  characters  and  relations  resemble 
vertebral  bodies,  and  there  are  good  reasons  for  believing  that  they 
represent  four  vertebrae  which,  in  later  stages,  are  taken  up  into  the 
skull  region  and  fuse  with  the  primitive  chondrocranium.  In  the 
human  embfyo  they  are  less  distinct  than  in  lower  mammals,  but  since 
a  three-rooted  hypoglossal  and  a  suboccipital  nerve  also  occur  in  man 
it  is  probable  that  the  corresponding  vertebrae  are  also  represented. 
Indeed,  confirmation  of  their  existence  may  be  found  in  the  fact 
that  during  the  cartilaginous  stage  of  the  skull  the  hypoglossal  fora- 
mina are  divided  into  three  portions  by  two  cartilaginous  partitions 
which  separate  the  three  roots  of  the  hypoglossal  nerve.  It  seems 
certain  from  the  evidence  derived  from  embryology  and  comparative 


170 


DEVELOPMENT    OF    THE    SKULL 


anatomy  that  the  human  skull  is  composed  of  a  primitive  unseg- 
mental  chondrocranium  plus  four  vertebrae,  the  latter  being  added 

to  and  incorporated  with  the  occip- 
ital portion  of  the  chondrocranium. 
Emphasis  must  be  laid  upon 
the  fact  that  the  cartilaginous  por- 
tion of  the  skull  forms  only  the 
base  and  lower  portions  of  the  sides 
of  the  cranium,  its  entire  roof,  as 
well  as  the  face  region,  showing  no 
indication  of  cartilage,  the  mesen- 
chyme in  these  regions  being  con- 
verted into  fibrous  connective  tissue, 
which,  especially  in  the  cranial  re- 
gion, assumes  the  form  of  a  dense 
membrane. 

But  in  addition  to  the  chondro- 
cranium and  the  vertebras  incorpo- 
rated with  it,  other  cartilaginous  ele- 
ments enter  into  the  composition  of 
the  skull.  The  mesenchyme  which 
occupies  the  axis  of  each  branchial 
arch  undergoes  more  or  less  com- 
plete chondrification,  cartilaginous 
bars  being  so  formed,  certain  of 
which  enter  into  very  close  rela- 
tions with  the  skull.  It  has  been 
seen  (p.  92)  that  each  half  of  the 
first  arch  gives  rise  to  a  maxillary 
process  which  grows  forward  and 
ventrally  to  form  the  anterior 
boundary  of  the  mouth,  while  the 
remaining  portion  of  the  arch  forms 
the  mandibular  process.  The 
whole  of  the  axis  of  the  mandib- 


J-^V:y: 


Fig.  104. — Frontal  Section 
through  the  occipital  and  upper 
Cervical  Regions  of  a  Calf  Embryo 
of  8.7  MM. 

ai  and  ai1,  Intervertebral  arteries; 
be1,  first  cervical  intervertebral  plate; 
bo,  suboccipital  intervertebral  plate; 
c1—  2,  cervical  nerves;  eh,  notochord; 
K,  vertebral  centrum;  ml— 3,  occipital 
myotomes;  m4— 5,  cervical  myotomes; 
01— 3,  roots  of  hypoglossal  nerve;  vj, 
jugular  vein;  x  and  xi,  vagus  and  spinal 
accessory  nerves. — (Froriep.) 


DEVELOPMENT    OF    THE    SKULL 


171 


ular  process  becomes  chondrified,  forming  a  rod  known  as  Meckel's 
cartilage,  and  this,  at  its  dorsal  end,  comes  into  relation  with  the 
periotic  capsule,  as  does  also  the  dorsal  end  of  the  cartilage  of 
the  second  arch.  In  the  remaining  three  arches  cartilage  forms 
only  in  the  ventral  portions,  so  that  their  rods  do  not  come  into 
relation  with  the  skull,  though  it  will  be  convenient  to  consider 
their  further  history  together  with  that  of  the  other  branchial  arch 
cartilages.  The  arrangement 
of  these  cartilages  is  shown  dia- 
grammatically  in  Fig.  105. 

By  the  ossification  of  these 
various  parts  three  categories  of 
bones  are  formed:  (1)  cartilage 
bones  formed  in  the  chondro- 
cranium,  (2)  membrane  bones, 
and  (3)  cartilage  bones  devel- 
oping from  the  cartilages  of  the 
branchial  arches.  The  bones 
belonging  to  each  of  these  cate- 
gories are  primarily  quite  distinct  from  one  another  and  from 
those  of  the  other  groups,  but  in  the  human  skull  a  very  consid- 
erable amount  of  fusion  of  the  primary  bones  takes  place,  and 
elements  belonging  to  two  or  even  to  all  three  categories  may  unite 
to  form  a  single  bone  of  the  adult  skull.  In  a  certain  region  of  the 
chondrocranium  also  and  in  one  of  the  branchial  arches  the  original 
cartilage  bone  becomes  ensheathed  by  membrane  bone  and  event- 
ually disappears  completely,  so  that  the  adult  bone,  although  repre- 
sented by  a  cartilage,  is  really  a  membrane  bone.  And,  indeed, 
this  process  has  proceeded  so  far  in  certain  portions  of  the  branchial 
arch  skeleton  that  the  original  cartilaginous  representatives  are 
no  longer  developed,  but  the  bones  are  deposited  directly  in  connec- 
tive tissue.  These  various  modifications  interfere  greatly  with  the 
precise  application  to  the  human  skull  of  the  classification  of  bones 
into  the  three  categories  given  above,  and  indeed  the  true  significance 
of  certain  of  the  skull  bones  can  only  be  perceived  by  comparative 


Fig.  105. — Diagram  showing  the  Five 

Branchial  Cartilages,  I  to'F. 

At,  Atlas;  Ax,  epistropheus;  3,  third 

cervical  vertebra. 


172 


OSSIFICATION    OF    THE    CH0NDR0CRANIUM 


studies.  Nevertheless  it  seems  advisable  to  retain  the  classification, 
indicating,  where  necessary,  the  confusion  of  bones  of  the  various 
categories. 

The  Ossification  of  the  Chondrocranium. — The  ossification 
of  the  cartilage  of  the  occipital  region  results  in  the  formation  of 
four  distinct  bones  which  even  at  birth  are  separated  from  one 

another  by  bands  of  cartilage. 
The  portion  of  cartilage  lying  in 
front  of  the  foramen  magnum 
ossifies  to  form  a  basioccipital 
bone  (Fig.  106,  bo),  the  portions 
on  either  side  of  this  give  rise  to 
the  two  exoccipitals  (eo),  which 
bear  the  condyles,  and  the  por- 
tion above  the  foramen  produces 
a  supraoccipital  (so),  which  repre- 
sents the  part  of  the  squamous 
portion  of  the  adult  bone  lying 
below  the  superior  nuchal  line. 
All  that  portion  of  the  bone 
which  lies  above  that  line  is 
composed  of  membrane  bone 
which  owes  its  origin  to  the 
fusion  of  two  or  sometimes  four 
centers  of  ossification,  appearing 
in  the  membranous  roof  of  the  embryonic  skull.  The  bone  so 
formed  (ip)  represents  the  interparietal  of  lower  vertebrates  and,  at 
an  early  stage,  unites  with  the  supraoccipital,  although  even  at 
birth  an  indication  of  the  line  of  union  of  the  two  parts  is  to  be  seen 
in  two  deep  incisions  at  the  sides  of  the  bone.  The  union  of  the 
exoccipitals  and  supraoccipital  takes  place  in  the  course  of  the  first 
or  second  year  after  birth,  but  the  basioccipital  does  not  fuse  with 
the  rest  of  the  bone  until  the  sixth  or  eighth  year.  It  will  be  noticed 
that  no  special  centers  occur  for  the  four  occipital  vertebrae,  these 
structures  having  become  completely  incorporated  in  the  chondro- 


Fig.  106. — Occipital  Bone  of  a  Fetus 

at  Term. 
bo,  Basioccipital;  eo,  exoccipital;  ip,  in- 
terparietal; so,  supraoccipital. 


OSSIFICATION   OF   THE   CHONDROCRANIUM  173 

cranium,  and  even  the  cartilaginous  partitions  which  divide  the 
hypoglossal  foramina  usually  disappear  during  the  process  of 
ossification. 

Two  pairs  of  centers  have  been  described  for  the  interparietal 
bone  and  it  has  been  claimed  that  the  deep  lateral  incisions  divide 
the  lower  pair,  so  that  when  the  incisions  meet  and  persist  as  the 
sutura  mendosa,  separating  the  so-called  inca  bone  from  the  rest  of 
the  occipital,  the  division  does  not  correspond  to  the  line  between 
the  supraoccipital  and  the  interparietal,  but  a  portion  of  the  latter 
bone  remains  in  connection  with  the  supraoccipital.  Mall,  how- 
ever, in  twenty  preparations,  found  but  a  single  pair  of  centers  for 
the  interparietal. 

Occasionally  an  additional  pair  of  small  centers  appear  for  the 
uppermost  angle  of  the  interparietal,  and  the  bones  formed  from 
them  may  remain  distinct  as  what  have  been  termed  fontanelle 
bones. 


Fig.  107. — Sphenoid  Bone  from  Embryo  of  3^  to  4  Months. 
The  parts  which  are  still  cartilaginous  are  represented  in  black,     as,  Alisphenoid ; 
b,  basisphenoid;  /,  lingula;  os,  orbitosphenoid ;  p,  internal  pterygoid  plate. — (Sappey.) 

In  the  sphenoidal  region  the  number  of  distinct  bones  which 
develop  is  much  greater  than  in  the  occipital  region.  At  the  begin- 
ning of  the  second  month  a  center  appears  in  each  of  the  cartilages 
which  represent  the  alisphenoids  (great  wings).  These  cartilages 
do  not,  however,  represent  the  entire  extent  of  the  great  wings  and 
their  ossification  gives  rise  only  to  those  portions  of  the  bone  in  the 
neighborhood  of  the  foramina  ovale  and  rotundum  and  to  the 
lateral  pterygoid  plates.  The  remaining  portions  of  the  wings,  the 
orbital  and  temporal  portions,  develop  as  membrane  bone  (Fawcett) 


174  OSSIFICATION    OF    THE    CHONDROCRANIUM 

and  early  unite  with  the  portions  formed  from  the  cartilage.  At 
the  end  of  the  second  month  a  center  appears  in  each  orbito sphenoid 
(lesser  wing)  cartilage  (Fig.  107,  os),  and  a  little  later  a  pair  of 
centers  (b),  placed  side  by  side,  are  developed  in  the  cartilage 
representing  the  posterior  portion  of  the  body;  together  these  form 
what  is  known  as  the  basisphenoid.  Still  later  a  center  appears  on 
either  side  of  the  basisphenoids  to  form  the  UngulcB  (I),  and  another 
pair  appears  in  the  anterior  part  of  the  cartilage,  between  the  orbito- 
sphenoids,  and  represent  the  presphenoid. 

In  addition  to  these  ten  centers  in  cartilage  and  the  membrane 
portion  of  the  alisphenoid,  two  other  membrane  bones  are  included 
in  the  adult  sphenoid.  Thus,  a  little  before  the  appearance  of  the 
center  for  the  alisphenoids  an  ossification  is  formed  in  the  mesen- 
chyme of  each  lateral  wall  of  the  posterior  part  of  the  nasal  cavity 
and  gives  rise  to  the  medial  lamina  of  the  pterygoid  process,  the 
mesenchyme  at  the  tip  of  the  ossification  condensing  to  form  a 
cartilaginous  hook-like  structure  over  which  the  tendon  of  the  tensor 
veli  palatini  plays.  This  cartilage  later  ossifies  to  form  the  pterygoid 
hamulus,  the  medial  pterygoid  lamina  being  thus  a  combination  of 
membrane  and  cartilage,  the  latter,  however,  being  a  secondary 
development  and  quite  independent  of  the  chondrocranium. 

By  the  sixth  month  the  lingular  have  fused  with  the  basisphenoid 
and  the  orbitosphenoids  with  the  presphenoid,  and  a  little  later  the 
basisphenoid  and  presphenoid  unite.  The  alisphenoids  and  medial 
pterygoid  laminae  remain  separate,  however,  until  after  birth,  fusing 
with  the  remaining  portions  of  the  adult  bone  during  the  first  year. 

The  cartilage  of  the  ethmoidal  region  of  the  chondrocranium 
forms  somewhat  later  than  the  other  portions  and  consists  at  first 
of  a  stout  median  mass  projecting  downward  and  forward  into  the 
nasal  process  (Fig.  108,  Ip),  and  two  lateral  masses  {lm),  situated  one 
on  either  side  in  the  mesenchyme  on  the  outer  side  of  each  olfactory 
pit.  Ossification  of  the  lateral  masses  or  ectethmoids  begins  rela- 
tively early,  but  it  appears  in  the  upper  part  of  the  median  cartilage 
only  after  birth,  producing  the  crista  galli  and  the  perpendicular 
plate,  which  together  form  what  is  termed  the  mesethmoid.     When 


OSSIFICATION   OF    THE    CHONDROCRANIUM 


175 


first  formed,  the  three  cartilages  are  quite  separate  from  one  another, 
the  olfactory  and  nasal  nerves  passing  down  between  them  to  the 
olfactory  pit,  but  later  trabecular  begin  to  extend  across  from 
the  mesethmoid  to  the  upper  part  of  the  ectethmoids  and  eventually 
form  a  fenestrated  horizontal  lamella  which  ossifies  to  form  the 
cribriform  plate. 

The  lower  part  of  the  median  cartilage  does  not  ossify,  but  a 
center  appears  on  each  side  of  the  median  line  in  the  mesenchyme 
behind  and  below  its  posterior  or  lower 
border.  From  these  centers  two  verti- 
cal bony  plates  develop  which  unite 
by  their  median  surfaces  below,  and 
above  invest  the  lower  border  of  the 
cartilage  and  form  the  vomer.  The 
portion  of  the  cartilage  which  is  thus 
invested  undergoes  resorption,  but  the 
more  anterior  portions  persist  to  form 
the  cartilaginous  septum  of  the  nose. 
The  vomer,  consequently,  is  not  really 
a  portion  of  the  chondrocranium,  but 
is  a  membrane  bone;  its  intimate 
relations  with  the  median  ethmoidal 
cartilage,  however,  make  it  convenient 
to  consider  it  in  this  place. 

When  first  formed,  the  ectethmoids  are  masses  of  spongy  bone 
and  show  no  indication  of  the  honeycombed  appearance  which  they 
present  in  the  adult  skull.  This  condition  is  produced  by  the 
absorption  of  the  bone  of  each  mass  by  evaginations  into  it  of  the 
mucous  membrane  lining  the  nasal  cavity.  This  same  process  also 
brings  about  the  formation  of  the  curved  plates  of  bone  which 
project  from  the  inner  surfaces  of  the  lateral  masses  and  are  known 
as  the  superior  and  middle  conchse  (turbinated  bones).  The  inferior 
and  sphenoidal  conchae  are  developed  from  special  centers,  but 
belong  to  the  same  category  as  the  others,  being  formed  from  por- 
tions  of   the   lateral   ethmoidal   cartilages   which   become   almost 


Fig.  108. — Anterior  Portion 
of  the  Base  of  the  Skull  of  a 
6  to  7  Months'  Embryo. 

The  shaded  parts  represent 
cartilage.  cp,  Cribriform  plate; 
hn,  lateral  mass  of  the  ethmoid; 
Ip,  perpendicular  plate;  of  optic 
foramen;  os,  orbitosphenoid. — 
(After  von  Spec.) 


176 


OSSIFICATION    OF   THE   CHONDROCRANIUM 


separated  at  an  early  stage  before  the  ossification  has  made  much 
progress.  Absorption  of  the  body  of  the  sphenoid  bone  to  form 
the  sphenoidal  cells,  of  the  frontal  to  form  the  frontal  sinuses,  and 
of  the  maxillaries  to  form  the  maxillary  antra  is  also  produced  by 
outgrowths  of  the  nasal  mucous  membrane,  all  these  cavities,  as 
well  as  the  ethmoidal  cells,  being  continuous  with  the  nasal  cavities 
and  lined  with  an  epithelium  which  is  continuous  with  the  mucous 
membrane  of  the  nose. 

In  the  lower  mammalia  the  erosion  of  the  mesial  surface  of  the 
ectethmoidal  cartilages  results,  as  a  rule,  in  the  formation  of  five  conchae, 
while  in  man  but  three  are  usually  recognized.  Not  infrequently, 
however,  the  human  middle  concha  shows  indications,  more  or  less 
marked,  of  a  division  into  an  upper  and  a  lower  portion,  which  corre- 
spond to  the  third  and  fourth  bones  of  the  typical  mammalian  arrange- 
ment.    Furthermore,  at  the  upper  portion  of  the  nasal  wall,  in  front  of 

the  superior  concha,  a  slight  elevation, 
termed  the  agger  nasi,  is  always  observa- 
ble, its  lower  edge  being  prolonged  down- 
ward to  form  what  is  termed  the  uncinate 
process  of  the  ethmoid.  This  process 
and  the  agger  together  represent  the  up- 
permost concha  of  the  typical  arrange- 
ment, to  which,  therefore,  the  human 
arrangement  may  be  reduced. 

A  number  of  centers  of  ossifica- 
tion— the  exact  number  is  yet  uncer- 
tain— appear  in  the  periotic  capsule 
during  the  later  portions  of  the  fifth 
month,  and  during  the  sixth  month 
these  unite  together  to  form  a  single 
center  from  which  the  complete  ossi- 
fication of  the  cartilage  proceeds  to  form  the  petrous  and  mastoid 
portions  of  the  temporal  bone  (Fig.  109,  p).  The  mastoid  process 
does  not  really  form  until  several  years  after  birth,  being  produced 
by  the  hollowing  and  bulging  out  of  a  portion  of  the  petrous  bone 
by  out-growths  from  the  lining  membrane  of  the  middle  ear.  The 
cavities  so  formed  are  the  mastoid  cells,  and  their  relations  to  the 
middle-ear  cavity  are  in  all  respects  similar  to  those  of  the  ethmoidal 


Fig.    109. — The    Temporal 

Bone  at  Birth.     The  Styloid 

Process  and  Auditory  Ossicles 

are  not  Represented. 

p,  Petrous  bone;  s,  squamosal; 

t,  tympanic. — (Poirier.) 


OSSIFICATION   OF   THE   CHONDROCRANIUM 


177 


and  sphenoidal  cells  to  the  nasal  cavities.  The  remaining  portions 
of  the  temporal  bone  are  partly  formed  by  membrane  bone  and 
partly  from  the  branchial  arch  skeleton.  An  ossification  appears  at 
the  close  of  the  eighth  week  in  the  membrane  which  forms  the  side 
of  the  skull  in  the  temporal  region  and  gives  rise  to  a  squamosal 
bone  (s),  which  later  unites  with  the  petrous  to  form  the  squamosal 
portion  of  the  adult  temporal,  and  another  membrane  bone,  the 
tympanic  (/),  develops  from  a  center  appearing  in  the  mesenchyme 
surrounding  the  external  auditory  meatus,  and  later  also  fuses  with 
the  petrous  to  form  the  floor  and  sides  of  the  external  meatus,  giving 
attachment  at  its  inner  edge  to  the  tympanic  membrane.  Finally, 
the  styloid  process  is  developed  from  the  upper  part  of  the  second 
branchial  arch,  whose  history  will  be  considered  later. 

The  various  ossifications  which  form  in  the  chondrocranium  and 
the  portions  of  the  adult  skull  which  represent  them  are  shown  in  the 
following  table: 


Region  of 
Chondrocranium. 


Ossification. 


IBasioccipital 
Exoccipitals 
Supraoccipital 


Sphenoidal 


Ethmoidal  . 


Basisphenoid 

Presphenoid 

Lingulae 

Alisphenoids 

Orbitosphenoids 

Mesethmoid 


Ectethmoids 


Parts  of  Adult  Skull; 

Basilar  process. 
Condyles. 

Squamous  portion  below  superior  nuchal 
line. 

Body. 

Greater  wings  (in  part) . 
Lesser  wings. 
Lamina  perpendicularis. 
Crista  galli. 
Nasal  septum. 
Lateral  masses. 
Superior  concha. 
Middle  concha. 


Inferior  concha. 

Sphenoidal  concha. 

,,    .    .  f  Mastoid. 

Penolic  capsule <  _. 

1  Petrous. 

The  Membrane  Bones  of  the  Skull.— In  the  membrane  form- 
ing the  sides  and  roof  of  the  skull  in  the  second  stage  of  its  develop- 


178  THE  MEMBRANE  BONES  OF  THE  SKULL 

ment  ossifications  appear,  which  give  rise,  in  addition  to  the  inter- 
parietal and  squamosal  bones  already  mentioned  in  connection  with 
the  occipital  and  temporal,  to  the  parietals  and  frontal.  Each  of  the 
former  bones  develops  from  a  single  center  which  appears  at  the 
end  of  the  eighth  week,  while  the  frontal  is  formed  at  about  the  same 
time  from  two  centers  situated  symmetrically  on  each  side  of  the 
median  line  and  eventually  fusing  completely  to  form  a  single  bone, 
although  more  or  less  distinct  indications  of  a  median  suture,  the 
metopic,  are  not  infrequently  present. 

Furthermore,  ossifications  appear  in  the  mesenchyme  of  the 
facial  region  to  form  the  nasal,  lachrymal,  and  zygomatic  bones,  all 
of  which  arise  from  single  centers  of  ossification.  In  the  case  of  each 
zygomatic  bone,  however,  three  osseous  thickenings  appear  on  the 
inner  surface  of  the  original  ossification,  which  then  disappears  and 
the  thickenings  unite  to  form  the  adult  bone,  though  occasionally 
one  or  more  of  their  lines  of  union  may  persist,  producing  a  bipartite 
or  tripartite  zygomatic. 

The  vomer,  which  has  already  been  described,  belongs  also 
strictly  to  the  category  of  membrane  bones,  as  do  also  the  maxillae 
and  the  palatines;  these  latter,  however,  primarily  belonging  to  the 
branchial  arch  skeleton,  with  which  they  will  be  considered. 

The  purely  membrane  bones  in  the  skull,  are,  then,  the  following: 

Interparietals Part  of  squamous  portion  of  occipital. 

Pterygoids Medial  pterygoid  plates. 

Squamosals Squamous    portions    of    temporals. 

Tympanies Tympanic  plates  of  temporals. 

Parietals. 

Frontal. 

Nasals. 

Lachrymals. 

Zygomatics. 

Vomer. 

The  Ossification  of  the  Branchial  Arch  Skeleton. — It  has 

been  seen  (p.  171)  that  a  cartilaginous  bar  develops  only  in  the 
mandibular  process  of  the  first  branchial  arch.  In  the  maxillary 
process  no  cartilaginous  skeleton  forms,  but  two  membrane  bones, 


OSSIFICATION    OF    BRANCHIAL  ARCH    SKELETON 


179 


Fig.  i  10. — Diagram  of  the  Ossi- 
fications of  which  the  Maxilla 
is  Composed,  as  seen  from  the 
Outer  Surface.  The  Arrow 
Passes  through  the  Infraor- 
bital Canal. — {From  von  Spee, 
after  Sappey.) 


the  palatine  and  maxilla,  are  developed  in  it,  their  cartilaginous 
representatives,  which  are  to  be  found  in  lower  vertebrates,  having 
been  suppressed  by  a  condensation  of  the  development.  The 
palatine  bone  develops  from  a  single  center  of  ossification,  but  for 
each  maxilla  no  less  than  five  centers  have  been  described  (Fig.  no). 
One  of  these  gives  rise  to  so  much  of  the  alveolar  border  of  the  bone 
as  contains  the  bicuspid  and  molar  teeth;  a  second  forms  the  nasal 
process  and  the  part  of  the  alveolar 
border  which  contains  the  canine 
tooth;  a  third  the  portion  which  con- 
tains the  incisor  teeth;  while  the 
fourth  and  fifth  centers  lie  above  the 
first  and  give  rise  to  the  inner  and 
outer  portions  of  the  orbital  plate 
and  the  body  of  the  bone.  The 
first,  second,  fourth,  and  fifth  por- 
tions early  unite  together,  but  the 
third  center,  which  really  lies  in  the 
ventral  part  of  the  nasal  process,  remains  separate  for  some  time, 
forming  what  is  termed  the  premaxilla,  a  bone  which  remains  per- 
manently distinct  in  the  majority  of  the  lower  mammals. 

The  above  is  the  generally  accepted  view  as  to  the  development  of 
the  maxilla.  Mall,  however,  maintains^  that  it  has  but  twro  centers  of 
ossification,  one  giving  rise  to  the  premaxilla  and  the  other  to  the  rest  of 
the  bone.  The  maxillary  center  makes  its  appearance  about  the  middle 
of  the  sixth  week. 

Since  the  condition  known  as  hare-lip  results  from  a  failure  of  the 
maxillary  process  to  unite  completely  with  the  frontonasal  process  (see 
p.  100),  and  since  the  premaxilla  develops  in  the  latter  and  the  maxilla 
in  the  former,  the  cleft  may  pass  between  these  two  bones  and  prevent 
their  union  (see  also  p.  284). 

The  upper  end  of  Meckel's  cartilage  passes  between  the  tympanic 
bone  and  the  outer  surface  of  the  periotic  capsule  and  thus  comes 
to  lie  apparently  within  the  tympanic  cavity  of  the  ear;  this  portion 
of  the  cartilage  divides  into  two  parts  which  ossify  to  form  two  of  the 
bones  of  the  middle  ear,  the  malleus  and  incus,  a  description  of 


i8o 


OSSIFICATION   OF   BRANCHIAL  ARCH   SKELETON 


whose  further  development  may  be  postponed  until  a  later  chapter. 
At  about  the  middle  of  the  sixth  week  of  development  a  plate  of 
membrane  bone  appears  to  the  outer  side  of  the  lower  portion  of  the 
cartilage  and  gradually  extends  to  form  the  body  and  ramus  of  the 
mandible. 

In  the  region  of  the  body  the  bone  develops  so  as  to  enclose  the 
cartilage,  together  with  the  inferior  alveolar  (dental)  nerve  which 
lies  to  the  outer  side  of  the  cartilage,  but  in  the  region  of  the  ramus 


Z.ChT 


Fig.  hi.— Model  of  Right  Half  of  Mandible  of  a  Fetus  95  mm.  in  Length, 
seen  from  the  mesial  surface. 
C1  and  C2,  Accessory  cartilages;  Ch.  T.,  chorda  tympanijO.,  cartilage  for  coronoid 
process;  Cy.,  cartilage  for  condyloid  process;  Mai.,  malleus;  M.C.,  Meckel's  cartilage; 
N.  Al.,  inferior  alveolar  nerve;  N.  Aur.,  auriculo-temporal  nerve;  N.L.,  lingual  nerve; 
N.Mh.,  mylo-hyoid  nerve;  N.T.,  trigeminal  nerve;  Sy.,  symphysis. — (Low.) 


the  bone  remains  entirely  to  the  outer  side  of  the  cartilage  and  nerve, 
whence  the  position  of  the  mandibular  foramen  on  the  inner  surface 
of  the  adult  bone.  The  anterior  portion  of  Meckel's  cartilage 
becomes  ossified  by  the  extension  of  ossification  from  the  membrane 
bone  into  it,  the  portion  corresponding  to  the  body  of  the  bone  behind 
the  mental  foramen  disappears  and  the  portion  above  the  mandibu- 
lar foramen  is  said  to  become  transformed  into  fibrous  connective 
tissue  and  to  persist  as  the  spheno-mandibular  ligament.  At  the 
upper  extremity  of  the  ramus  two  nodules  of  cartilage  develop,  quite 
independently,  however,  of  Meckel's  cartilage  (Fig.  in,  Cr  and  Cy), 


OSSIFICATION    OF    BRANCHIAL   ARCH  SKELETON 


181 


and  ossification  extends  into  these  from  the  ramus  to  form  the 
coronoid  and  condyloid  processes.  And,  finally,  two  other  inde- 
pendent cartilages  appear  toward  the  anterior  extremity  of  each  half 


Fig.  112. — Diagram  showing  the  Categories  to  which  the  Bones  of  the  Skull 
.       .  Belong. 

The  unshaded  bones  are  membrane  bones,  the  heavily  shaded  represent  the 
chondrocranium,  while  the  black  represents  the  branchial  arch  elements.  AS,  Ali- 
sphenoid;  ExO,  exoccipital;  F,  frontal;  Hy,  hyoid;  IP,  interparietal;  Z,  zygomatic; 
Mn,  mandible;  Mx,  maxilla;  NA,  nasal;  P,  parietal;  Pe,  periotic;  SO,  supraoccipital; 
Sg,  squamosal;  St,  styloid  process;  Th,  thyreoid  cartilage;  Ty,  tympanic. 


of  the  bone,  one  at  the  alveolar  (Ct)  and  the  other  at  the  lower 
border  (C2),  and  these,  also  are  later  incorporated  into  the  bone 
without  developing  special  centers  of  ossification. 


182  OSSIFICATION   OF    BRANCHIAL  ARCH   SKELETON 

Each  half  of  the  mandible  thus  ossifies  from  a  single  center,  and 
is  essentially  a  membrane  bone  replacing  a  cartilaginous  precursor. 
At  birth  the  two  halves  are  united  at  the  symphysis  by  fibrous  tissue, 
into  which  ossification  extends  later,  union  occurring  in  the  first 
or  second  year. 

The  upper  part  of  the  cartilage  of  the  second  branchial  arch  also 
comes  into  relation  with  the  tympanic  cavity  and  ossifies  to  form  the 
styloid  process  of  the  temporal  bone.  The  succeeding  moiety  of  the 
cartilage  undergoes  degeneration  to  form  the  stylo-hyoid  ligament, 
while  its  most  ventral  portion  ossifies  as  the  lesser  comu  of  trie  hyoid 
bone.  The  great  variability  which  may  be  observed  in  the  length 
of  the  styloid  processes  and  of  the  lesser  cornua  of  the  hyoid  depends 
upon  the  extent  to  which  the  ossification  of  the  original  cartilage 
proceeds,  the  length  of  the  stylo-hyoid  ligaments  being  in  inverse 
ratio  to  the  length  of  the  processes  or  cornua.  The  greater  cornua 
of  the  hyoid  are  formed  by  the  ossification  of  the  cartilages  of  the 
third  arch,  and  the  body  of  the  bone  is  formed  from  a  cartilaginous 
plate,  the  copula,  which  unites  the  ventral  ends  of  the  two  arches 
concerned. 

Finally,  the  cartilages  of  the  fourth  and  fifth  branchial  arches 
early  fuse  together  to  form  a  plate  of  cartilage,  and  the  two  plates 
of  opposite  sides  unite  by  their  ventral  edges  to  form  the  thyreoid 
cartilage  of  the  larynx. 

The  accompanying  diagram  (Fig.  112)  shows  the  various  struc- 
tures derived  from  the  branchial  arch  skeleton,  as  well  as  some  of 
the  other  elements  of  the  skull,  and  a  re'sume'  of  the  fate  of  the  bran- 
chial arches  may  be  stated  in  tabular  form  as  follows,  the  parts  repre- 
sented by  cartilage  which  becomes  replaced  by  membrane  bone 
being  printed  in  italics,  while  membrane  bones  which  have  no 
cartilaginous  representatives  are  enclosed  in  brackets: 

(Maxilla). 

(Palatine) . 

Malleus. 

Incus. 

Spheno-mandibular  ligament. 

Mandible. 


1st  arch. 


DEVELOPMENT    OF   APPENDICULAR    SKELETON  1 83 

(Styloid  process  of  the  temporal. 
Stylo-hyoid  ligament. 
Lesser  cornu  of  hyoi<  1 . 

3d  arch Greater  cornu  of  hyoid. 

4th  and  5th  arches Thyreoid  cartilage  of  larynx. 

The  Development  of  the  Appendicular  Skeleton. — While 
the  greater  portion  of  the  axial  skeleton  is  formed  from  the  sclero- 
tomes of  the  mesodermic  somites,  the  appendicular  skeleton  is 
derived  from  the  somatic  mesenchyme,  which  is  not  divided  into 
metameres.  This  mesenchyme  forms  the  core  of  the  limb  bud  and 
becomes  converted  into  cartilage,  by  the  ossification  of  which  all  the 
bones  of  the  limbs,  with  the  possible  exception  of  the  clavicle,  are 
formed. 

Of  the  bones  of  the  pectoral  girdle  the  clavicle  requires  further 
study  before  it  can  be  certain  whether  it  is  to  be  regarded  as  a  purely 
cartilage  bone  or  as  a  combination  of  cartilage  and  membrane 
ossification  (Gegenbaur).  It  is  the  first  bone  of  the  skeleton  to 
ossify,  two  centers  appearing  for  each  bone  at  about  the  sixth  week 
of  development.  The  tissue  in  which  the  ossifications  form  has 
certain  peculiar  characters,  and  it  is  difficult  to  say  whether  it  is  to  be 
regarded  as  cartilage  which,  on  account  of  the  early  differentiation 
of  the  center,  has  not  yet  become  thoroughly  differentiated  histologic- 
ally, or  as  some  other  form  of  connective  tissue.  However  that  may 
be,  true  cartilage  develops  on  either  side  of  the  ossifying  region,  and 
into  this  the  ossification  gradually  extends,  so  that  at  least  a  portion 
of  the  bone  is  preformed  in  cartilage. 

The  scapula  is  at  first  a  single  plate  of  cartilage  in  which  two 
centers  of  ossification  appear.  One  of  these  gives  rise  to  the  body 
and  the  spine,  while  the  other  produces  the  coracoid  process  (Fig. 
113,  co),  the  rudimentary  representative  of  the  coracoid  bone  which 
extends  between  the  scapula  and  sternum  in  the  lower  vertebrates. 
The  coracoid  does  not  unite  with  the  body  until  about  the  fifteenth 
year,  and  secondary  centers  which  give  rise  to  the  vertebral  edge  (b) 
and  inferior  angle  of  the  bone  (a)  and  to  the  acromion  process  (c) 
unite  with  the  rest  of  the  bone  at  about  the  twentieth  year. 


1 84 


DEVELOPMENT    OF   APPENDICULAR    SKELETON 


The  humerus  and  the  bones  of  the  forearm  are  typical  long  bones, 
each  of  which  develops  from  a  primary  center,  which  gives  rise  to 
the  shaft,  and  has,  in  addition,  two  or  more  epiphysial  centers.  In 
the  humerus  an  epiphysial  center  appears  for  the  head,  another  for 
the  greater  tuberosity,  and  usually  a  third  for  the  lesser  tuberosity, 
while  at  the  distal  end  there  is  a  center  for  each  condyle,  one  for  the 
trochlea  and  one  for  the  capitulum,  the  fusion  of  these  various 
epiphyses  with  the  shaft  taking  place  between  the  seventeenth  and 


Fig.  113. — The  Ossification  Cen- 
ters of  the  Scapula. 
a,  b,  and  c,  Secondary  centers  for 
the  angle,  vertebral  border,  and  acro- 
mion; co,  center  for  the  coracoid  proc- 
ess.— (Testut.) 


Fig.  114. — Reconstruction  of  an 
Embryonic  Carpus. 

c,  Centrale;  cu,  triquetral;  lu,  lunate; 
m,  capitate;  p,  pisiform;  sc,  navicular;  t, 
greater  multangular;  tr,  lesser  multangular; 
u,  hamate. 


twentieth  years.  The  radius  and  ulna  each  possesses  a  single  epi- 
physial center  for  each  extremity  in  addition  to  the  primary  center 
for  the  shaft,  the  proximal  epiphysial  center  for  the  ulna  giving 
rise  to  the  tip  of  the  olecranon  process. 

The  embryological  development  of  the  carpus  is  somewhat 
complicated.  A  cartilage  is  found  representing  each  of  the  bones 
normally  occurring  in  the  adult  (Fig.  114),  and  these  are  arranged 
in  two  distinct  rows:  a  proximal  one  consisting  of  three  elements, 


DEVELOPMENT    OF   APPENDICULAR   SKELETON  185 

named  from  their  relation  to  the  bones  of  the  forearm,  radiate, 
intermedium,  and  ulnar e;  and  a  distal  on^composed  of  four  elements, 
termed  carpalia.  In  addition,  a  cartilage,  termed  the  pisiform,  is 
found  on  the  ulnar  side  of  the  proximal  row  ^nd  is  generally  j^g&rded 
as  a  sesamoid  cartilage  developed  in  the /tendon  of  the  flei 
ulnaris,  and  furthermore  a  number  of  inconstant  carti 
been  observed  whose  significance  in  the  majority  of  cast 
less  obscure.  These  accessory  cartilage^either  disappc 
stages  of  development  or  fuse  with  neighboring  cartilages^ 
cases,  ossify  and  form  distinct  elements  of  the  carpus, 
however,  occurs  so  frequently  as  almdK  to  deserve^  classification  as 
a  constant  element;  it  \p  known  asvthje  ceniraie  (Fig.  114,  c)  and 
occupies  a  position  between  the/car\ua!§;es  of  the  proximal  and  distal 
rows  and  apparently  correspond  ~r&.  a  cartilage  typVally  present 
in  lower  forms  and  o^fying*to~f»rai  a  distinct  bone.  Iri  tha  human 
carpus  its  fate  varies,  wfe  it  may\eitnfer  disappear  or  unitp  with  other 
cartilages,  that  with  wpich  it  most  usually  fuses  b'eing  probably  the 
radiale.  There  is  evraence  also  to  sfrftw  that  another  ofJthe  accessory 
cartilages  unites/with  the  ulnar  element  of  the  distatsAw,  represent- 
ing the  carpale  v  typically  present  in  lower  forms. 

Each  of  the  eleinents  corresponding  to  an  adu^t)  bone  ossifies 

from  a  single  centerwith  the  exception  of  carpale  iv-Xwhich  has  two 
centers,  a  furtherindication  of  its  composite  character.  The  rela- 
tion of  the  cartrteg&s  to  the  adult  bones  may  be  seen  from  the  table 
given  on  page  loX^J  \v_^ 

With  regard  toYhe  metacarpals  and  phalanges;  it  need  merely 
be  stated  that  each  develops  from  a  single  primary  center  for  the 
shaft  and  one  secondary  epiphysial  center.  The"  primary  center 
appears  at  about  the  middle  of  the  shaft  excepJ  in  the  terminal 
phalanges,  in  which  it  appears  at  the  distal  enfr  of  the  cartilage. 
The  epiphyses  for  the  metacarpals  are  at  the  distends  of  the  bones, 
except  in  the  case  of  the  metacarpal  of  the  ihumb,  which  resembles 
the  phalanges  in  having  its  epiphysis  at  the  proximal  end. 

Each  innominate  bone  appears  as  a  somewhat  oval  plate  of 
cartilage  whose  long  axis  is  directed  almost  at  right  angles  to  the 


i86 


DEVELOPMENT    OF   APPENDICULAR    SKELETON 


vertebral  column  and  which  is  in  close  relation  with  the  fourth  and 
fifth  sacral  vertebrae.  As  development  proceeds  a  rotation  of  the 
cartilage,  accompanied  by  a  slight  shifting  of  position,  occurs,  so 
that  eventually  the  plate  has  its  long  axis  almost  parallel  with  the 
vertebral  column  and  is  in  relation  with  the  first  three  sacrals. 
Ossification  appears  at  three  points  in  each  cartilage,  one  in  the 

upper  part  to  form  the  ilium  (Fig. 
115,  il)  and  two  in  the  lower  part, 
the  anterior  of  these  giving  rise  to 
the  pubis  (p),  while  the  posterior 
produces  the  ischium  (is).  At 
birth  these  three  bones  are  still 
separated  from  one  another  by  a 
Y-shaped  piece  of  cartilage  whose 
three  limbs  meet  at  the  bottom 
of  the  acetabulum,  but  later  a 
secondary  center  appears  in  this 
cartilage  and  unites  the  three 
bones  together.  The  central  part 
of  the  lower  half  of  each  original 
cartilage  plate  does  not  undergo 
complete  chondrification,  but  re- 
mains membranous,  constituting 
the  obturator  membrane  which 
closes  the  obturator  foramen. 
In  addition  to  the  Y-shaped  secondary  center,  other  epiphysial 
centers  appear  in  the  prominent  portions  of  the  cartilage,  such  as 
the  pubic  crest  (Fig.  115,  c),  the  ischial  tuberosity  (d),  the  anterior 
inferior  spine  (b)  and  the  crest  of  the  ilium  (a),  and  unite  with  the 
rest  of  the  bone  at  about  the  twentieth  year. 

The  femur,  tibia,  and  fibula  each  develop  from  a  single  primary 
center  for  the  shaft  and  an  upper  and  a  lower  epiphysial  center,  the 
femur  possessing,  in  addition,  epiphysial  centers  for  the  greater 
and  lesser  trochanters  (Fig.  94).  The  patella  does  not  belong  to 
the  same  category  as  the  other  bones,  but  resembles  the  pisiform 


Fig.  115. — The  Ossification  Centers 
of  the  os  innominatum. 
a,  b,  c,  and  d,  Secondary  centers  for 
the  crest,  anterior  inferior  spine,  sym- 
physis, and  ischial  tuberosity;  il,  ilium; 
is,  ischium;  p,  pubis. — (Testut.) 


DEVELOPMENT    OF   APPENDICULAR   SKELETON 


l87 


bone  of  the  carpus  in  being  a  sesamoid  bone,  developed  in  the  tendon 
of  the  quadriceps  extensor  cruris.  Its  cartilage  does  not  appear 
until  the  fourth  month  of  intrauterine  life,  when  most  of  the  primary 
centers  for  other  bones  have  already  appeared,  and  its  ossification 
does  not  begin  until  the  third  year  after  birth. 

The  tarsus,  like  the  carpus,  consists  of  a  proximal  row  of  three 
cartilages,  termed  the  tibiale,  the  intermedium,  and  the  fibulare,  and 
of  a  distal  row  of  four  tarsalia.  Between  these  two  rows  a  single 
cartilage,  the  centrale,  is  interposed.  Each  of  these  cartilages  ossifies 
from  a  single  center,  that  of  the  intermedium  early  fusing  with  the 
tibiale,  though  it  occasionally  remains  distinct  as  the  os  trigonum,  and 
from  a  comparison  with  lower  forms  it  seems  probable  that  the 
fibular  cartilage  of  the  distal  row  really  represents  two  separate 
elements,  there  being,  properly  speaking,  five  tarsalia  instead  ot 
four.  The  fibulare,  in  addition  to  its  primary  center,  possesses  also 
an  epiphysial  center,  which  develops  at  the  point  of  insertion  of  the 
tendo  Achillis. 

A  comparison  of  the  carpal  and  tarsal  cartilages  and  their 
relations  to  the  adult  bones  may  be  seen  from  the  following  table: 


Carpus 

Tarsus 

Cartilages 

Bones 

Bones 

Cartilages 

Radiale 

Navicular 

Talus 

f  Tibiale 

\  Intermedium 

Intermedium 

Lunate 

Ulnare 

Triquetral 

Calcaneus 

Fibulare 

Sesamoid  cartilage 

Pisiform 



— — 

Centrale 

Fuses  with  navicular 

Navicular 

Centrale 

Carpale     I 

Gr.  multangular 

1  st  Cuneiform 

Tarsale      I 

Carpale    II 

Less,  multangular 

2d  Cuneiform 

Tarsale    II 

Carpale  III 

Capitate 

3d  Cuneiform 

Tarsale  III 

Carpale  IV  1 
Carpale    V  J 

Hamate 

Cuboid 

(  Tarsale  TV 
I  Tarsale    V 

1 88  DEVELOPMENT    OF    THE    JOINTS 

The  development  of  the  metatarsals  and  phalanges  is  exactly 
similar  to  that  of  the  corresponding  bones  of  the  hand  (see  p.  185). 

The  Development  of  the  Joints. — The  mesenchyme  which 
primarily  represents  each,  vertebra,  or  the  skull,  or  the  skeleton  of 
a  limb,  is  at  first  a  continuous  mass,  and  when  it  becomes  converted 
into  cartilage  this  also  may  be  continuous,  as  in  the  skull,  or  may 
appear  as  a  number  of  distinct  parts  united  by  unmodified  portions 
of  the  mesenchyme.  In  the  former  case  the  various  ossifications 
as  they  extend  will  come  into  contact  with  their  neighbors  and  will 
either  fuse  with  them  or  will  articulate  with  them  directly,  forming 
a  suture. 

When,  however,  a  portion  of  unmodified  mesenchyme  intervenes 
between  two  cartilages,  the  mode  of  articulation  of  the  bones  formed 
from  these  cartilages  will  vary.  The  intermediate  mesenchyme 
may  in  time  undergo  chondrification  and  unite  the  bones  in  an 
almost  immovable  articulation  known  as  a  synchondrosis  (e.  g.,  the 
articulation  of  the  first  rib  with  the  sternum) ;  or  a  cavity  may  appear 
in  the  center  of  the  intervening  cartilage  so  that  a  slight  amount  of 
movement  of  the  two  bones  is  possible,  forming  an  amphiar  thro  sis 
(e.  g.,  the  symphysis  pubis);  or,  finally,  the  intermediate  mesen- 
chyme may  not  chondrify,  but  its  peripheral  portions  may  become 
converted  into  a  dense  sheath  of  connective  tissue  (Fig.  116,  c) 
which  surrounds  the  adjacent  ends  of  the  two  bones  like  a  sleeve, 
forming  the  articular  capsule,  while  the  central  portions  degenerate 
to  form  a  cavity.  The  bones  which  enter  into  such  an  articulation 
are  more  or  less  freely  movable  upon  one  another  and  the  joint  is 
termed  a  diarthrosis  (e.  g.,  the  knee-  or  shoulder-joint). 

In  a  diarthrosis  the  connective-tissue  cells  near  the  inner  surface 
of  the  capsule  arrange  themselves  in  a  layer  to  form  a  synovial 
membrane  for  the  joint,  and  portions  of  the  capsule  may  thicken 
to  form  special  bands,  the  reinforcing  ligaments,  while  other  strong 
fibrous  bands,  which  may  pass  from  one  bone  to  the  other,  forming 
accessory  ligaments,  are  shown  by  comparative  studies  to  be  in  many 
cases  degenerated  portions  of  what  were  originally  muscles. 

In  certain  diarthroses,  such  as  the  temporo-mandibular  and 


DEVELOPMENT    OF    THE    JOINTS  189 

sternoclavicular,  the  whole  of  the  central  portions  of  the  inter- 
mediate mesenchyme  does  not  degenerate,  but  it  is  converted  into  a 
fibro-cartilage,  between  each  surface  of  which  and  the  adjacent 
bone  there  is  a  cavity.  These  interarticular  cartilages  seem,  in  the 
sterno-clavicular  joints,  to  represent  the  sternal  ends  of  a  bone 
existing  in  lower  vertebrates  and  known  as  the  precoracoid,  but  it 
seems   doubtful   if   those   of   the   temporo-mandibular  and  knee- 


Fig.  116.— Longitudinal  Section  through  the  Joint  oe  the  Great  Toe  in  an 

Embryo  of  4.5  cm. 
c,  Articular  capsule;  i,  intermediate  mesenchyme  which  has  almost  disappeared  in  the 
center;  p1  and  p2,  cartilages  of  the  first  and  second  phalanges. — (Nicholas.) 

joints  have  a  similar  significance,  the  most  recent  observations  on 
their  development  tending  to  derive  them  from  the  intermediate 
mesenchyme. 

From  their  mode  of  development  it  is  evident  that  the  cavities  of 
diarthrodial  joints  are  completely  closed  and  their  walls,  except  where 
they  are  formed  by  cartilage,  are  lined  by  a  continuous  layer  of  synovial 
cells.  Ligaments  or  tendons,  which,  at  first  sight,  appear  to  traverse  the 
cavities  of  certain  joints,  are  in  reality  excluded  from  them,  being  lined 
by  a  sheath  of  synovial  cells  continuous  with  the  layer  fining  the  general 
cavity.  Thus,  the  tendon  of  the  long  head  of  the  biceps,  which  seems  to 
traverse  the  shoulder-joint  is,  in  the  fetus,  entirely  outside  the  articular 
capsule,  upon  which  it  rests.  Later  it  sinks  in  toward  the  joint  cavity, 
pushing  the  articular  capsule  before  it,  so  that  it  lies  at  first  in  a  groove 
in  the  capsule,  which  later  on  becomes  converted  into  a  canal  and,  finally, 
separates  from  the  rest  of  the  capsule  except  at  its  two  extremities, 


190  LITERATURE 

forming  a  cylindrical  canal,  open  at  either  end,  traversing  the  joint  cavity 
and  containing  the  tendon  of  the  biceps. 

The  ligamentum  teres  of  the  hip-joint  is  similarly  excluded  from  the 
joint  cavity  by  a  sheath  of  synovium,  which  extends  outward  around  it 
from  the  bottom  of  the  acetabular  fossa  to  the  depression  in  the  head  of 
the  femur,  and  in  the  knee-joint  the  crucial  ligaments  are  also  excluded 
from  the  cavity  by  a  reflection  of  the  synovium.  This  joint,  indeed,  is 
in  the  fetus  incompletely  divided  into  two  parts,  one  corresponding  to 
each  femoral  condyle,  by  a  partition  which  extends  backward  from  the 
patellar  ligament  to  the  crucial  ligaments,  remains  of  this  partition 
persisting  in  the  adult  as  the  so-called  ligamentum  mucosum. 


LITERATURE. 

C.  R.  Bardeen:  "  The  Development  of  the  Thoracic  Vertebrae  in  Man,"  Amer.  Journ. 

Anat.,  iv,  1905. 
C.  R.  Bardeen:  "Studies  of   the    Development  of   the   Human   Skeleton,"    Amer 

Journ.  Anat.  iv,  1905. 
C.  R.  Bardeen:  "Early  Development  of  the  Cervical  Vertebra  and  the  Base  of  the 

Occipital  Bone  in  Man,"  Amer.  Journ.  Anat.,  vm,  1908. 
C.  R.  Bardeen:  "Vertebral  Regional  Determination  in  Young  Human  Embryos," 

Anat.  Record,  11,  1908. 
E.  T.  Bell:  "On  the  Histogenesis  of  the  Adipose  Tissue  of  the  Ox,"  Amer.  Journ. 

Anat.,  ix,  1909. 
A.  Bernays:  "Die    Entwicklungsgeschichte    des    Kniegelenks    des    Menschen    mit 

Bemerkungen  liber  die  Gelenke  im  Allgemeinen,"  Morpholog.  Jahrbuch,  TV,  1878. 
E.  Dtjrsy:  "Zur  Entwicklungsgeschichte  des  Kopfes  des  Menschen  und  der  hoheren 

Wirbelthiere,"  Tubingen,  1869. 
E.  Fawcett:  "On  the  Development,  Ossification  and  Growth  of  the  Palate  Bone," 

Journ.  Anat.  and  Phys.,  XL,  1906. 
E.  Fawcett:  "Notes  on  the  Development  of  the  Human  Sphenoid,"  Journ.  Anat. 

and  Phys.,  xliv,  1910. 
E.  Fawcett:  "The  Development  of  the  Human  Maxilla,  Vomer  and  Paraseptal  Car- 
tilages," Journ.  Anat.  and  Phys.,  xlv,  1911. 
A.  Froriep:  "Zur  Entwicklungsgeschichte  der  Wirbelsaule,  insbesondere  des  Atlas 

und  Epistropheus  und  der  Occipitalregion,"  Archiv  fur  Anat.  und  Physiol.,  Anat. 

Abth.,  1886. 
E.  Gaupp:  "Alte  Probleme  und  neuere  Arbeiten  iiber  den  Wirbeltierschadel,"  Ergeb. 

der  Anat.  und  Entwicklungsgesch.,  x,  1901. 
C.  Gegenbaur:  "Ein  Fall  von  erblichem  Mangel  der  Pars  acromialis  Claviculae,  mit 

Bemerkungen  iiber  die  Entwicklung  der  Clavicula,"  Jenaische  Zeitschr.filr  medic. 

Wissensch.,  I,  1864. 
J.  Golowinski:  "Zur  Kenntnis  der  His.togenese  der  Bindegewebsfibrillen,"  Anat. 

Hefte,  xxxiii,  1907. 


LITERATURE  -    191 

E.  Grafenberg:  "Die  Entwirklung  der  Knochen,  Muskeln  unci  Nerven  der  Hand  und 

der  fur  die  Bewegungen  der  Hand  bestimmten  Muskeln  des  Unterarms,"  Anat. 

Hefte,  xxx,  1906. 
Henkeand  Reyher:  "Studien  liber  die  Entwickelung  der  Extremitaten  des  Menschen, 

insbesondere  der  Gelenkflachen,"  Sitzungsberichte  der  kais.  Akad.  Wien,  LXX,  1875. 
M.  Jakoby:  "Beitrag  zur  Kenntnis  des  menschlichen  Primordialcraniums,"  Archiv 

fiir  mikrosk.  Anat.,  xliv,  1894. 
K.  Kjellberg:  "Beitrage  zur  Entwicklungsgeschichte  des  Kiefergelenks,"  Morph. 

Jahrbuch,  xxxii,  1904. 
H.    Leboucq:  "Recherches   sur   la   morphologie   du   carpe   chez   les   mammiferes," 

Archives  de  Biolog.,  V,  1884. 
G.  Levi:  "Beitrag  zum  Studium  der  Entwickelung  des  knorpeligen  Primordialcran- 
iums des  Menschen,"  Archiv  fiir  mikrosk.  Anat.,  lv,  1900. 
A.  Linck:  "Beitrage  zur  Kennlnis  der  menschlichen  Chorda  dorsalis  in  Hals-  und 

Kopfskelett,  etc.,"  Anat.  Hefte,  xlii,  1911. 
A.  Low:  "Further  Observations  on  the  Ossification  of  the  Human  Lower  Jaw," 

Journ.  Anat.  and  Phys.,  xliv,  1910. 
M.  Lucien:  "  Developpement  de  l'articulation  du  genou  et  formation  du  ligament 

adipeux,"  Bibliogr.  Anat.,  xiii,  1904. 

F.  P.  Mall:  "The  Development  of  the  Connective  Tissues  from  the  Connective-tissue 

Syncytium,"  Amer.  Jour.  Anat.,  1,  1902. 
F.  P.  Mall:  "On  Ossification  Centers  in  Human  Embryos  Less  Than  One  Hundred 
Days  Old,"  Amer.  Journ.  Anat.,  V  1906. 

F.  Merkel:  "Betrachtungen  fiber  die  Entwicklung  des  Bindegewebes,"  Anat.  Hefte, 

xxxviii,  1909. 
W.  van  Noorden:  "Beitrag  zur  Anatomie  der  knorpeligen  Schadelbasis  menschlicher 

Embryonen,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1887. 
A.  M.  Paterson:  "The  Human  Sternum,"  Liverpool,  1904. 
K.  Peter:  "  Anlage  und  Homologie  der  Muscheln  des  Menschen  und  der  Saugetiere," 

Arch,  fur  mikrosk.  Anat.,  lx,  1902. 
J.  W.  Pryor:  "The  Chronology  and  Order  of  Ossification  of  the  Bones  of  the  Human 

Carpus,"  Bulletin  State  Univ.,  Lexington,  Ky.,  1908. 
Rambaut  et  Renault:  "Origine  et  developpement  des  Os,"  Paris,  1864. 
E.  Rosenberg:  "Ueber  die  Entwickelung  der  Wirbelsaule  und  das  Centrale  carpi  des 

Menschen,"  Morpholog.  Jahrbuch,  1,  1876. 
H.  and  H.  Rouviere:  "Sur  le  developpement  de  l'antre  mastoidien  et  les  cellules 

mastoidiennes,"  Bibliogr.  Anat.,  xx,  1910. 

G.  Ruge:  "  Untersuchungen  liber  die  Entwickelungsvorgange  am  Brustbein  des 
Menschen,"  Morpholog.  Jahrbuch,  VI,  1880. 

J.  P.  Schaffer:  "The  Lateral  Wall  of  the  Cavum  Nasi  in  Man,  with  Especial 
Reference  to  the  Various  Developmental  Stages,"  Journ.  Morph.,  xxi,  1910. 

J.  P.  Schaffer:  "The  Sinus  Maxillaris  and  its  Relations  in  the  Embryo,  Child  and 
Adult  Man,"  Amer  Journ.  Anat.,  x,  1910. 

G.  Thilenius:  "Untersuchungen  iiber  die  morphologische  Bedeutung  accessorischer 
Elemente  am  menschlichen  Carpus  (und  Tarsus),"  Morpholog.  Arbeiten,  V,  1896. 


192  LITERATURE 

K.  Toldt  Jr.:  "Entwicklung  und  Struktur  des  menschlichen  Jochbeines," Sitzungsber. 

k.  Acad.  Wissensch.  Wien,  M ath.-naturwiss  Kl.,  Cxi,  1902. 
A.  Vinogradoff:  "Developpement  de  l'articulation  temporo-maxillaire  chez  l'homme 

dans  la  periode  intrauterine,"  Internal.  Monatsschr.  Anat.  Phys.,  xxvil,  1910. 
R.  H.  Whitehead  and  J.  A.  Waddell:  "The  Early  Development  of  the  Mammalian 

Sternum,"  Amer.  Journ.  Anat.,  xii,  191 1. 
L.  W.  Williams:  "The  Later  Development  of  the  Notochord,"  Amer.  Journ.  Anat., 

vin,  1908. 
E.   Zuckerkandl:  "Ueber  den   Jacobsonschen  Knorpel  und   die   Ossifikation   des 

Pflugscharbeines,"  Sitzb.  Akad.  Wiss.  Wien.,  cxvn,  1908. 


CHAPTER  VIII. 
THE  DEVELOPMENT   OF  THE  MUSCULAR   SYSTEM. 

Two  forms  of  muscular  tissue  exist  in  the  human  body,  the 
striated  tissue,  which  forms  the  skeletal  muscles  and  is  under  the 
control  of  the  central  nervous  system,  and  the  non-striated,  which  is 
controlled  by  the  sympathetic  nervous  system  and  is  found  in  the 
skin,  in  the  walls  of  the  digestive  tract,  the  blood-vessels  and  lym- 
phatics, and  in  connection  with  the  genito-urinary  apparatus.  In 
the  walls  of  the  heart  a  muscle  tissue  occurs  which  is  frequently 
regarded  as  a  third  form,  characterized  by  being  under  control  of 
the  sympathetic  system  and  yet  being  striated;  it  is,  however,  in  its 
origin  much  more  nearly  allied  to  the  non-striated  than  to  the 
striated  form  of  tissue,  and  will  be  considered  a  variety  of  the  former. 

The  Histogenesis  of  Non-striated  Muscular  Tissue. — With 
the  exception  of  the  sphincter  and  dilator  of  the  pupil  and  the  muscles 
of  the  sudoriparous  glands,  which  are  formed  from  the  ectoderm, 
all  the  non-striated  muscle  tissue  of  the  body  is  formed  by  the  con- 
version of  mesenchyme  cells  into  muscle-fibers.  The  details  of 
this  process  have  been  worked  out  by  McGill  for  the  musculature 
of  the  digestive  and  respiratory  tracts  of  the  pig  and  are  as  follows: 
The  mesenchyme  surrounding  the  mucosa  in  these  tracts  is  at  first 
a  loose  syncytium  (Fig.  117,  m)  and  in  the  regions  where  the  muscle 
tissue  is  to  form  a  condensation  of  the  mesenchyme  occurs  followed 
by  an  elongation  of  the  mesenchyme  cells  and  their  nuclei,  so  that 
the  muscle  layers  become  clearly  distinguishable  from  the  neighbor- 
ing undifferentiated  tissue  (Fig.  117,  mm).  Fibrils  of  two  kinds 
then  begin  to  appear  in  the  cytoplasm  of  the  muscle  cells.  Coarse 
fibrils  (f.c)  make  their  appearance  as  rows  of  granules,  which  enlarge 
and  increase  in  number  until  they  finally  fuse  to  form  homogeneous 
13  i93 


194         HYSTOGENESIS    OF    NON-STRIATED    MUSCULAR   TISSUE 


mm. 


7.nz. 


Fig.  117. — Longitudinal  Section  of  the  Lower  Part  of  the  Oesophagus  of  a 
Pig  Embryo  of  15  mm,  Showing  the  Histogenesis  of  the  Non-striated 
Musculature. 

b,  Basement  membrane;  e,  epithelium; /.c,  coarse  fibril;//.,  fine  fibril;  ga,  ganglion 
of  Auerbach's  plexus;  gm,  ganglion  of  Meissner's  plexus;  m,  mesenchyne;  mm, 
muscularis  mucosae;  pb,  protoplasmic  bridge;  vf,  varicose  fibril. — (McCill.) 


HISTOGENESIS    OF    NON-STRIATED    MUSCULAR    TISSUE 


J95 


fibrils  that  are  at  first  varicose,  but  later  become  of  uniform  caliber. 
Fine  fibrils  (/./)  which  are  homogeneous  from  the  first,  make  their 
appearance  after  the  coarse  ones  and  in  some  cases  seem  to  be 
formed  by  the  splitting  of  the  latter.  They  are  scattered  uniformly 
throughout  the  cytoplasm  of  the  muscle  cells  and  increase  in  number 
as  development  proceeds,  while  the  coarse  fibrils  diminish  and  may 
be  entirely  wanting  in  the  adult  tissue. 

Some  of  the  mesenchyme  cells  in  each  muscle  sheet  fail  to 
undergo  the  differentiation  just  described  and  multiply  to  form  the 
interstitial  connective  tissue, 
which  usually  divides  the  mus- 
cle cells  into  more  or  less  dis- 
tinct bundles.  Traces  of  the 
original  syncytial  nature  of 
the  tissue  are  to  be  seen  in 
the  intercellular  bridges  that 
occur  between  the  non-striated 
muscle  cells  of  many  adult 
forms. 

The  cells  from  which  the 
heart  musculature  develops 
are  at  first  of  the  usual  well 
defined  embryonic  type,  but, 
as  development  proceeds,  they 
become  irregularly  stellate  in 
form,  the  processes  of  neighbor- 
ing cells  fuse  and,  eventually, 
there  is  formed  a  continuous 
mass  of  protoplasm  or  syncytium  in  which  all  traces  of  cell  bounda- 
ries are  lacking  (Fig.  118).  While  the  individual  cells,  or  myoblasts 
as  they  are  termed,  are  still  recognizable,  granules  appear  in  their 
cytoplasm,  and  these  arrange  themselves  in  rows  and  unite  to  form 
slender  fibrils,  which  at  first  do  not  extend  beyond  the  limits  of  the 
myoblasts  in  which  they  have  appeared,  but  later,  as  the  fusion  of  the 
cells  proceeds,  are  continued  from  one  cell  territory  into  the  other 


Fig.  118. — Section  through  the  Heart- 
wall  of  a  Duck  Embryo  of  Three  Days. 
— (M.  Heidenhain.) 


196 


HISTOGENESIS    OF    NON-STRIATED    MUSCULAR   TISSUE 


through  considerable  stretches  of  the  syncytium,  without  regard  to 
the  original  cell  areas. 

The  fibrils  multiply,  apparently  by  longitudinal  division,  and 
arrange  themselves  in  circles  around  areas  of  the  syncytium  (com- 
pare Fig.  119).  As  the  multiplication  of  the  fibrils  continues  those 
newly  formed  arrange  themselves  around  the  interior  of  each  of  the 
original  circles  and  gradually  occupy  the  entire  cytoplasm,  or  sarco- 
plasm  as  it  may  now  be  termed,  except  immediately  around  the  nuclei 
where,  even  in  the  adult,  a  certain  amount  of  undifferentiated  sarco- 
plasm  persists.     The  fibrils  when  first  formed  are  apparently  homo- 


Fig.  119. — Cross-section  of  a  Muscle  prom  the  Thigh  of  a  Pig  Embryo  75  mm. 

Long. 
A,  Central  nucleus;  B,  new  peripheral  nucleus. — (Macallum.) 


geneous,  but  later  they  become  differentiated  into  two  distinct  sub- 
stances which  alternate  with  one  another  throughout  the  length 
of  the  fibril  and  produce  the  characteristic  transverse  striation  of  the 
tissue.  Finally  stronger  interrupted  transverse  bands  of  so-called 
cement  substance  appear,  dividing  the  tissue  into  areas  which  have 
usually  been  regarded  as  representing  the  original  myoblasts,  but 
are  really  devoid  of  significance  as  cells,  the  tissue  remaining, 
strictly  speaking,  a  syncytium. 


HISTOGENESIS    OF    STRIATED    MUSCLE    TISSUE  197 

The  Histogenesis  of  Striated  Muscle  Tissue.— The  histo- 
genesis of  striated  or  voluntary  muscle  tissue  resembles  very  closely 
that  which  has  just  been  described  for  the  heart  muscle.  There  is  a 
similar  formation  of  a  syncytium  by  the  fusion  of  the  cells  of  the 
myotomes,  an  appearance  of  granules  which  unite  to  form  fibrils, 
an  increase  of  the  fibrils  by  longitudinal  division  and  a  primary 
arrangement  of  the  fibrils  around  the  periphery  of  areas  of  sarco- 
plasm  (Fig.  119),  each  of  which  represents  a  muscle  fiber.  In 
addition  there  is  an  active  proliferation  of  the  nuclei  of  the  original 
myoblasts,  the  new  nuclei  arranging  themselves  more  or  less  regu- 
larly in  rows  and  later  migrating  from  their  original  central  position 
to  the  periphery  of  the  fibers,  and,  in  the  limb  muscles,  the  develop- 
ment is  further  complicated  by  a  process  of  degeneration  which 
affects  groups  of  muscle  fibers,  so  that  bundles  of  normal  fibers  are 
separated  by  strands  of  degenerated  tissue  in  which  the  fibrils  have 
disappeared,  the  nuclei  have  become  pale  and  the  sarcoplasm  vacuo- 
lated and  homogeneous.  Later  the  degenerated  tissue  seems  to 
disappear  entirely  and  mesenchymatous  connective  tissue  grows  in 
between  the  persisting  fibers,  grouping  them  into  bundles  and  the 
bundles  into  the  individual  muscles. 

So  long  as  the  formation  of  new  fibrils  continues,  the  increase  in 
the  thickness  of  a  muscle  is  probably  due  to  a  certain  extent  to  an 
increase  in  the  actual  number  of  fibers,  which  results  from  the  divi- 
sion of  those  already  existing.  Subsequently,  however,  this  mode  of 
growth  ceases,  the  further  increase  of  the  muscle  depending  upon  an 
increase  in  size  of  its  constituent  elements  (Macallum). 

The  Development  of  the  Skeletal  Muscles. — It  has  already 
been  pointed  out  that  all  the  skeletal  muscles  of  the  body,  with  the 
exception  of  those  connected  with  the  branchial  arches,  are  derived 
from  the  myotomes  of  the  mesodermic  somites,  even  the  limb 
muscles  possibly  having  such  an  origin,  although  the  cells  of  the 
tissue  from  which  the  muscles  of  the  limb  buds  form  lack  an  epithe- 
lial arrangement  and  are  indistinguishable  from  the  somatic  mesen- 
chyme which  forms  the  axial  cores  of  the  limbs. 

The  various  fibers  of  each  myotome  are  at  first  loosely  arranged, 


I98  DEVELOPMENT    OF    SKELETAL   MUSCLES 

but  later  they  become  more  compact  and  are  arranged  parallel  with 
one  another,  their  long  axes  being  directed  antero-posteriorly. 
This  stage  is  also  transitory,  however,  the  fibers  of  each  myotome 
undergoing  various  modifications  to  produce  the  conditions  existing 
in  the  adult,  in  which  the  original  segmental  arrangement  of  the 
fibers  can  be  perceived  in  comparatively  few  muscles.  The  exact 
nature  of  these  modifications  is  almost  unknown  from  direct  obser- 
vation, but  since  the  relation  between  a  nerve  and  the  myotome 
belonging  to  the  same  segment  is  established  at  a  very  early  period 
of  development  and  persists  throughout  life,  no  matter  what  changes 
of  fusion,  splitting,  or  migration  the  myotome  may  undergo,  it  is 
possible  to  trace  out  more  or  less  completely  the  history  of  the  various 
myotomes  by  determining  their  segmental  innervation.  It  is  known, 
for  example,  that  the  latissimus  dorsi  arises  from  the  seventh  and 
eighth*  cervical  myotomes,  but  later  undergoes  a  migration,  becom- 
ing attached  to  the  lower  thoracic  and  lumbar  vertebrae  and  to  the 
crest  of  the  ilium,  far  away  from  its  place  of  origin  (Mall),  and  yet 
it  retains  its  nerve-supply  from  the  seventh  and  eighth  cervical 
nerves  with  which  it  was  originally  associated,  its  nerve-supply 
consequently  indicating  the  extent  of  its  migration. 

By  following  the  indications  thus  afforded,  it  may  be  seen  that 
the  changes  which  occur  in  the  myotomes  may  be  referred  to  one  or 
more  of  the  following  processes: 

1.  A  longitudinal  splitting  into  two  or  more  portions,  a  process 
well  illustrated  by  the  trapezius  and  sternomastoid,  which  have 
differentiated  by  the  longitudinal  splitting  of  a  single  sheet  and 
contain  therefore  portions  of  the  same  myotomes.  The  sterno- 
hyoid and  omohyoid  have  also  differentiated  by  the  same  process, 
and,  indeed,  it  is  of  frequent  occurrence. 

2.  A  tangential  splitting  into  two  or  more  layers.  Examples  of 
this  are  also  abundant  and  are  afforded  by  the  muscles  of  the  fourth, 
fifth,  and  sixth  layers  of  the  back,  as  recognized  in  English  text-books 

*  This  enumeration  is  based  on  convenience  in  associating  the  myotomes  with  the 
nerves  which  supply  them.  The  myotomes  mentioned  are  those  which  correspond  to 
the  sixth  and  seventh  cervical  vertebrae. 


DEVELOPMENT  OF  SKELETAL  MUSCLES  1 99 

of  anatomy,  by  the  two  oblique  and  the  transverse  layers  of  the 
abdominal  walls,  and  by  the  intercostal  muscles  and  the  transversus 
of  the  thorax. 

3.  A  fusion  of  portions  of  successive  myotomes  to  form  a  single 
muscle,  again  a  process  of  frequent  occurrence,  and  well  illustrated 
by  the  rectus  abdominis  (which  is  formed  by  the  fusion  of  the 
ventral  portions  of  the  last  six  or  seven  thoracic  myotomes)  or  by 
the  superficial  portions  of  the  sacro-spinalis. 

4.  A  migration  of  parts  of  one  or  more  myotomes  over  others. 
An  example  of  this  process  is  to  be  found  in  the  latissimus  dorsi, 
whose  history  has  already  been  referred  to,  and  it  is  also  beautifully 
shown  by  the  serratus  anterior  and  the  trapezius,  both  of  which  have 
extended  far  beyond  the  limits  of  the  segments  from  which  they  are 
derived. 

5.  A  degeneration  of  portions  or  the  whole  of  a  myotome. 
This  process  has  played  a  very  considerable  part  in  the  evolution 
of  the  muscular  system  in  the  vertebrates.  When  a  muscle  nor- 
mally degenerates,  it  becomes  converted  into  connective  tissue,  and 
many  of  the  strong  aponeurotic  sheets  which  occur  in  the  body  owe 
their  origin  to  this  process.  Thus,  for  example,  the  aponeurosis 
connecting  the  occipital  and  frontal  portions  of  the  occipito-frontalis 
is  formed  in  this  process  and  is  muscular  in  such  forms  as  the  lower 
monkeys,  and  a  good  example  is  also  to  be  found  in  the  aponeurosis 
which  occupies  the  interval  between  the  superior  and  inferior 
serrati  postici,  these  two  muscles  being  continuous  in  lower  forms. 
The  strong  lumbar  aponeurosis  and  the  aponeuroses  of  the  oblique 
and  transverse  muscles  of  the  abdomen  are  also  good  examples. 

Indeed,  in  comparing  one  of  the  mammals  with  a  member  of 
one  of  the  lower  classes  of  vertebrates,  the  greater  amount  of  con- 
nective tissue  compared  with  the  amount  of  muscular  tissue  in  the 
former  is  very  striking,  the  inference  being  that  these  connective- 
tissue  structures  (fasciae,  aponeuroses,  ligaments)  represent  portions 
of  the  muscular  tissue  of  the  lower  form  (Bardeleben).  Many  of  the 
accessory  ligaments  occurring  in  connection  with  diarthrodial  joints 
apparently  owe  their  origin  to  a  degeneration  of  muscle  tissue,  the 


200  THE    TRUNK   MUSCULATURE 

fibular  lateral  ligament  of  the  knee-joint,  for  instance,  being  probably 
a  degenerated  portion  of  the  peroneus  longus,  while  the  sacro- 
tuberous  ligament  appears  to  stand  in  a  similar  relation  to  the  long 
head  of  the  biceps  femoris  (Sutton). 

6.  Finally,  there  may  be  associated  with  any  of  the  first  four 
processes  a  change  in  the  direction  of  the  muscle-fibers.  The 
original  antero-posterior  direction  of  the  fibers  is  retained  in  com- 
paratively few  of  the  adult  muscles  and  excellent  examples  of  the 
process  here  referred  to  are  to  be  found  in  the  intercostal  muscles 
and  the  muscles  of  the  abdominal  walls.  In  the  musculature 
associated  with  the  branchial  arches  the  alteration  in  the  direction 
of  the  fibers  occurs  even  in  the  fishes,  in  which  the  original  direction 
of  the  muscle-fibers  is  very  perfectly  retained  in  other  myotomes,  the 
branchial  muscles,  however,  being  arranged  parallel  with  the 
branchial  cartilages  or  even  passing  dorso-ventrally  between  the 
upper  and  lower  portions  of  an  arch,  and  so  forming  what  may  be 
regarded  as  a  constrictor  of  the  arch.  This  alteration  of  direction 
dates  back  so  far  that  the  constrictor  arrangement  may  well  be 
taken  as  the  primary  condition  in  studying  the  changes  which  the 
branchial  musculature  has  undergone  in  the  mammalia. 

It  would  occupy  too  much  space  'in  a  work  of  this  kind  to  con- 
sider in  detail  the  history  of  each  one  of  the  skeletal  muscles  of  the 
human  body,  but  a  statement  of  the  general  plan  of  their  develop- 
ment will  not  be  out  of  place.  For  convenience  the  entire  system 
may  be  divided  into  three  portions — the  cranial,  trunk  and  limb 
musculature;  and  of  these,  the  trunk  musculature  may  first  be 
considered. 

The  Trunk  Musculature. — It  has  already  been  seen  (p.  82) 
that  the  myotomes  at  first  occupy  a  dorsal  position,  becoming 
prolonged  ventrally  as  development  proceeds  so  as  to  overlap  the 
somatic  mesoderm,  until  those  of  opposite  sides  come  into  contact 
in  the  mid-ventral  line.  Before  this  is  accomplished,  however,  a 
longitudinal  splitting  of  each  myotome  occurs,  whereby  there  is 
separated  off  a  dorsal  portion  which  gives  rise  to  a  segment  of  the 
dorsal  musculature  of  the  trunk  and  is  supplied  by  the  ramus  dorsalis 


THE    TRUNK    MUSCULATURE  201 

of  its  corresponding  spinal  nerve.  In  the  lower  vertebrates  this 
separation  of  each  of  the  trunk  myotomes  into  a  dorsal  and  ventral 
portion  is  much  more  distinct  in  the  adult  than  it  is  in  man,  the  two 
portions  being  separated  by  a  horizontal  plate  of  connective  tissue 
extending  the  entire  length  of  the  trunk  and  being  attached  by  its 
inner  edge  to  the  transverse  processes  of  the  vertebrae,  while  per- 
ipherally it  becomes  continuous  with  the  connective  tissue  of  the 


Fig.  120. — Embryo  of  13  mm.  showing  the  Formation  of  the  Rectus  Muscle.— 

{Mall.) 

dermis  along  a  line  known  as  the  lateral  line.  In  man  the  dorsal 
portion  is  also  much  smaller  in  proportion  to  the  ventral  portion 
than  in  the  lower  vertebrates.  From  this  dorsal  portion  of  the 
myotomes  are  derived  the  muscles  belonging  to  the  three  deepest 
layers  of  the  dorsal  musculature,  the  more  superficial  layers  being 


202  THE    TRUNK   MUSCULATURE 

composed  of  muscles  belonging  to  the  limb  system.  Further 
longitudinal  and  tangential  divisions  and  a  fusion  of  successive 
myotomes  bring  about  the  conditions  which  obtain  in  the  adult 
dorsal  musculature. 

While  the  myotomes  are  still  some  distance  from  the  mid-ventral 
line  another  longitudinal  division  affects  their  ventral  edges  (Fig. 
120),  portions  being  thus  separated  which  later  fuse  more  or  less 
perfectly  to  form  longitudinal  bands  of  muscle,  those  of  opposite 
sides  being  brought  into  apposition  along  the  mid-ventral  line  by 
the  continued  growth  ventrally  of  the  myotomes.  In  this  way  are 
formed  the  rectus  and  pyramidalis  muscles  of  the  abdomen  and  the 
depressors  of  the  hyoid  bone,  the  genio-hyoid  and  genio-glossus* 
in  the  neck  region.  In  the  thoracic  region  this  rectus  set  of  muscles, 
as  it  may  be  termed,  is  not  represented  except  as  an  anomaly,  its 
absence  being  probably  correlated  with  the  development  of  the 
sternum  in  this  region. 

The  lateral  portions  of  the  myotomes  which  intervene  between 
the  dorsal  and  rectus  muscles  divide  tangentially,  producing  from 
their  dorsal  portions  in  the  cervical  and  lumbar  regions  muscles, 
such  as  the  longus  capitis  and  colli  and  the  psoas,  which  lie  beneath 
the  vertebral  column  and  hence  have  been  termed  hyposkeletal 
muscles  (Huxley).  More  ventrally  three  sheets  of  muscles,  lying 
one  above  the  other,  are  formed,  the  fibers  of  each  sheet  being 
arranged  in  a  definite  direction  differing  from  that  found  in  the  other 
sheets.  In  the  abdomen  there  are  thus  formed  the  two  oblique  and 
the  transverse  muscles,  in  the  thorax  the  intercostals  and  the  trans- 
versa thoracis,  while  in  the  neck  these  portions  of  some  of  the  myo- 
tomes disappear,  those  of  the  remainder  giving  rise  to  the  scaleni 
muscles,  portions  of  the  trapezius  and  sternomastoid  (Bolk),  and 
possibly  the  hyoglossus  and  styloglossus.  In  the  abdominal  region, 
and  to  a  considerable  extent  in  the  neck  also,  the  various  portions  of 
myotomes  fuse  together,  but  in  the  thorax  they  retain  in  the  inter- 
costals their  original  distinctness,  being  separated  by  the  ribs. 

*  This  muscle  is  supplied  by  the  hypoglossal  nerve,  but  for  the  present  purpose  it  is 
convenient  to  regard  this  as  a  spinal  nerve,  as  indeed  it  primarily  is. 


THE    TRUNK    MUSCULATURE 


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204 


THE    TRUNK   MUSCULATURE 


The  table  on  page  203  will  show  the  relation  of  the  various  trunk 
muscles  to  the  portions  of  the  myotomes. 

The  intimate  association  between  the  pelvic  girdle  and  the  axial 
skeleton  brings  about  extensive  modifications  of  the  posterior  trunk 
myotomes.  So  far  as  their  dorsal  portions  are  concerned  probably 
all  these  myotomes  as  far  back  as  the  fifth  sacral  are  represented  in 
the  sacro-spinalis,  but  the  ventral  portions  from  the  first  lumbar 
myotome  onward  are  greatly  modified.  The  last  myotome  taking 
part  in  the  formation  of  the  rectus  abdominis  is  the  twelfth  thoracic 
and  the  last  to  be  represented    in  the  lateral  musculature  of  the 


A  B 

Fig.  121. — Perineal  Region  of  Embryos  of  (A)  Two  Months  and  (25)  Four  to 

Five  Months,  showing  the  Development  of  the  Perineal  Muscles. 

dc,  Nervus  dorsalis  clitoridis;  p,  pudendal  nerve;  sa,  sphincter  ani;  sc  sphincter  cloacae; 

sv,  sphincter  vaginse. — {Popowsky.) 

abdomen  is  the  first  lumbar,  the  ventral  portions  of  the  remaining 
lumbar  and  of  the  first  and  second  sacral  myotomes  either  having 
disappeared  or  being  devoted  to  the  formation  of  the  musculature 
of  the  lower  limb. 

The  ventral  portions  of  the  third  and  fourth  sacral  myotomes  are 
represented,  however,  by  the  levator  ani  and  coccygeus,  and  are  the 
last  myotomes  which  persist  as  muscles  in  the  human  body,  although 
traces  of  still  more  posterior  myotomes  are  to  be  found  in  muscles 
such  as  the  curvator  coccygis  sometimes  developed  in  connection 
with  the  coccygeal  vertebrae. 

The  perineal  muscles  and  the  external  sphincter  ani  are  also 


THE    CRANIAL   MUSCULATURE  205 

developments  of  the  third  and  fourth  (and  second)  sacral  myotomes. 
At  a  time  when  the  cloaca  (see  p.  280)  is  still  present,  a  sheet  of 
muscles  lying  close  beneath  the  integument  forms  a  sphincter  around 
its  opening  (Fig.  121).  On  the  development  of  the  partition  which 
divides  the  cloaca  into  rectal  and  urinogenital  portions,  the  sphincter 
is  also  diyided,  its  more  posterior  portion  persisting  as  the  external 
sphincter  ani,  while  the  anterior  part  gradually  differentiates  into  the 
various  perineal  muscles  (Popowsky). 

The  Cranial  Musculature. — As  was  pointed  out  in  an  earlier 
chapter,  the  existence  of  distinct  mesodermic  somites  has  not  yet 
been  completely  demonstrated  in  the  head  of  the  human  embryo, 
but  in  lower  forms,  such  as  the  elasmobranch  fishes,  they  are  clearly 
distinguishable,  and  it  may  be  supposed  that  their  indistinctness  in 
man  is  a  secondary  condition.  Exactly  how  many  of  these  somites 
are  represented  in  the  mammalian  head  it  is  impossible  to  say,  but 
it  seems  probable,  from  comparison  with  lower  forms,  that  there  is 
a  considerable  number.  The  majority  of  them,  however,  early 
undergo  degeneration,  and  in  the  adult  condition  only  three  are 
recognizable,  two  of  which  are  prseoral  in  position  and  one  postoral. 
The  myotomes  of  the  anterior  praeoral  segment  give  rise  to  the 
muscles  of  the  eye  supplied  by  the  third  cranial  nerve,  those  of  the 
posterior  one  furnish  the  superior  oblique  muscles  innervated  by  the 
fourth  nerve,  while  from  the  postoral  myotomes  the  lateral  recti, 
supplied  by  the  sixth  nerve,  are  developed.  The  muscles  sup- 
plied by  the  hypoglossal  nerve  are  also  derived  from  myotomes,  but 
they  have  already  been  considered  in  connection  with  the  trunk 
musculature. 

The  remaining  muscles  of  the  head  differ  from  all  other  voluntary 
muscles  of  the  body  in  the  fact  that  they  are  derived  from  the 
branchiomeres  formed  by  the  segmentation  of  the  cephalic  ventral 
mesoderm.  These  muscles,  therefore,  are  not  to  be  regarded  as 
equivalent  to  the  myotomic  muscles  if  their  embryological  origin  is 
to  be  taken  as  a  criterion  of  equivalency,  and  in  their  case  it  would 
seem,  from  the  fact  that  they  are  innervated  by  nerves  fundamentally 
distinct  from  those  which  supply  the  myotomic  muscles,  that  this 


2o6  THE    CRANIAL   MUSCULATURE 

criterion  is  a  good  one.  They  must  be  regarded,  therefore,  as 
belonging  to  a  special  category,  and  may  be  termed  branchiomeric 
muscles  to  distinguish  them  from  the  myotomic  set. 

If  their  embryological  origin  be  taken  as  a  basis  for  homology,  it  is 
clear  that  they  should  be  regarded  as  equivalent  to  the  muscles  derived 
from  the  ventral  mesoderm  of  the  trunk,  and  these,  as  has  been  seen, 
are  the  non-striated  muscles  associated  with  the  viscera,  among  which 
may  be  included  the  striated  heart  muscle.  At  first  sight  this  homology 
seems  decidedly  strained,  chiefly  because  long-continued  custom  has 
regarded  the  histological  and  physiological  peculiarities  of  striated  and 
non-striated  muscle  tissue  as  fundamental.  It  may  be  pointed  out, 
however,  that  the  branchiomeric  muscles  are,  strictly  speaking,  visceral 
muscles,  and  indeed  give  rise  to  muscle  sheets  (the  constrictors  of  the 
pharynx)  which  surround  the  upper  or  pharyngeal  portion  of  the  digestive 
tract.  It  is  possible,  then,  that  the  homology  is  not  so  strained  as  might 
appear,  but  further  discussion  of  it  may  profitably  be  deferred  until  the 
cranial  nerves  are  under  consideration. 

The  skeleton  of  the  first  branchial  arch  becomes  converted  partly 
into  the  jaw  apparatus  and  partly  into  auditory  ossicles,  and  the 
muscles  derived  from  the  corresponding  branchiomere  become 
the  muscles  of  mastication  (the  temporal,  masseter,  and  pterygoids), 
the  mylohyoid,  anterior  belly  of  the  digastric,  the  tensor  veli  palatini 
and  the  tensor  tympani.  The  nerve  which  corresponds  to  the  first 
branchial  arch  is  the  trigeminus  or  fifth,  and  consequently  these 
various  muscles  are  supplied  by  it. 

The  second  arch  has  corresponding  to  it  the  seventh  nerve,  and 
its  musculature  is  partly  represented  by  the  stylohyoid  and  posterior 
belly  of  the  digastric  and  by  the  stapedius  muscle  of  the  middle  ear. 
From  the  more  superficial  portions  of  the  branchiomere,  however,  a 
sheet  of  tissue  arises  which  gradually  extends  upward  and  downward 
to  form  a  thin  covering  for  the  entire  head  and  neck,  its  lower  portion 
giving  rise  to  the  platysma  and  the  nuchal  fascia  which  extends 
backward  from  the  dorsal  border  of  this  muscle,  while  its  upper  parts 
become  the  occipito-frontalis  and  the  superficial  muscles  of  the  face 
(the  muscles  of  expression),  together  with  the  fascia?  which  unite 
the  various  muscles  of  this  group.  The  extension  of  the 
platysma  sheet  of  muscles  over  the  face  is   well  shown  by  the 


THE    CRANIAL    MUSCULATURE 


207 


Fig.  122. — Head  of  Embryos  (.4)  of  Two  Months  and  (B)  of  Three 
Months  showing  the  Extension  of  the  Seventh  Nerve  upon  the  Face. — 
(Popowsky.) 


208  THE   CRANIAL   MUSCULATURE 

development  of  the  branches  of  the  facial  nerve  which  supply  it 
(Fig.  122). 

The  degeneration  of  the  upper  part  of  the  third  arch  produces  a 
shifting  forward  of  one  of  the  muscles  derived  from  its  branchiomere, 
the  stylopharyngeus  arising  from  the  base  of  the  styloid  process. 
The  innervation  of  this  muscle  by  the  ninth  nerve  indicates,  however, 
its  true  significance,  and  since  fibers  of  this  nerve  of  the  third  arch 
also  pass  to  the  constrictor  muscles  of  the  pharynx,  a  portion  of 
these  must  also  be  regarded  as  having  arisen  from  the  third 
branchiomere. 

The  cartilages  of  the  fourth  and  fifth  arches  enter  into  the  forma- 
tion of  the  larynx  and  the  muscles  of  the  corresponding  branchio- 
meres  constitute  the  muscles  of  the  larynx,  together  with  the  remain- 
ing portions  of  the  constrictors  of  the  pharynx  and  the  muscles  of 
the  soft  palate,  with  the  exception  of  the  tensor.  Both  these  arches 
have  branches  of  the  tenth  nerve  associated  with  them  and  hence 
this  nerve  supplies  the  muscles  named.  In  addition,  two  of  the 
extrinsic  muscles  of  the  tongue,  the  glosso-palatinus  and  chon- 
droglossus,  belong  to  the  fourth  or  fifth  branchiomere,  although 
the  remaining  muscles  of  this  physiological  set  are  myotomic  in 
origin. 

Finally,  portions  of  two  other  muscles  should  probably  be 
included  in  the  list  of  branchiomeric  muscles,  these  muscles  being 
the  trapezius  and  sternomastoid.  It  has  already  been  seen  that 
they  are  partly  derived  from  the  cervical  myotomes,  but  they  are 
also  innervated  in  part  by  the  spinal  accessory,  and  since  the  motor 
fibers  of  this  nerve  are  serially  homologous  with  those  of  the  vagus 
it  would  seem  that  the  muscles  which  they  supply  are  probably 
branchiomeric  in  origin.  Observations  on  the  development  of 
these  muscles,  determining  their  relations  to  the  branchiomeres, 
are  necessary,  however,  before  their  morphological  significance  can 
be  regarded  as  definitely  settled. 

The  table  on  page  209  shows  the  relations  of  the  various  cranial 
muscles  to  the  myotomes  and  branchiomeres,  as  well  as  to  the  motor 
cranial  nerves. 


THE    CRANIAL   MUSCULATURE 


209 


Eleventh 

Trapezius. 
Sterno- 
mastoid. 

Tenth 

Constric- 
tors of 
pharynx 
(in  part). 
Pharyngo- 
palatinus. 
Levator  veli 
palatini. 
Musculus 

uvulae. 
Muscles  of 
the    larynx. 
Glosso-pal- 

atinus. 

Chrondro- 

glossus. 

■5 
.S 

Stylo-pha- 

ryngeus. 

Constrictors 

of  pharynx 

(in  part). 

6 

> 
<U 
CO 

Stylohyoid. 

Digastric 

(posterior 

belly). 

Stapedius. 
Platysma. 
Occipito- 
frontalis. 

Muscles  of 

expression. 

CO 

a)  3 

h-1      M 

3 

Temporal. 
.  Masseter. 

Pterygoids. 

Mylohyoid. 

Digastric 

(anterior 

belly). 

Tensor  veli 

palatini. 

Tensor 

tympani. 

3 

3 
0 

O    u 
CO     O 

•A          <u 
3 

Superior 
Inferior 
Medial     _ 
Inferior  0 

0 

> 

M 
V 

1 

O 

«5 
<u 
"0 

s 

0 

'in 
U 

%  I 

u 

pq 

I 

3 

1 

14 


2IO  THE    LIMB    MUSCLES 

The  Limb  Muscles. — It  has  been  customary  to  regard  the  limb 
muscles  as  derivatives  of  certain  of  the  myotomes,  these  structures 
in  their  growth  vent  rally  in  the  trunk  walls  being  supposed  to  pass 
out  upon  the  postaxial  surface  of  the  limb  buds  and  loop  back  again 
to  the  trunk  along  the  praeaxial  surface,  each  myotome  thus  giving 
rise  to  a  portion  of  both  the  dorsal  and  the  ventral  musculature  of 
the  limb.  This  view  has  not,  however,  been  verified  by  direct 
observation  of  an  actual  looping  of  the  myotomes  over  the  axis  of 
the  limb  buds;  indeed,  on  the  contrary,  the  limb  muscles  have  been 
found  to  develop  from  the  cores  of  mesenchyme  which  form  the 
axes  of  the  limb  buds  and  from  which  the  limb  skeleton  is  also 
developed.  This  may  be  explained  by  supposing  that  the  limb 
muscles  are  primarily  derivatives  of  the  myotomes  and  that  an 
extensive  concentration  of  their  developmental  history  has  taken 
place,  so  that  the  axial  mesenchyme  actually  represents  myotomic 
material  even  though  no  direct  connection  between  it  and  the 
myotomes  can  be  discovered.  Condensations  of  the  developmental 
history  certainly  occur  and  the  fact  that  the  muscles  of  the  human 
limbs,  as  they  differentiate  from  the  axial  cores,  present  essentially 
the  same  arrangement  as  in  the  adult  seems  to  indicate  that  there  is 
actually  an  extensive  condensation  of  the  phylogenetic  history  of  the 
individual  muscles,  since  comparative  anatomy  shows  the  arrange- 
ment of  the  muscles  of  the  higher  mammalian  limbs  to  be  the  result 
of  a  long  series  of  progressive  modifications  from  a  primitive  condi- 
tion. However,  even  though  this  be  the  case,  there  is  yet  the 
possibility  that  the  limb  musculature,  like  the  limb  skeleton,  may 
take  its  origin  from  the  ventral  mesoderm  and  consequently  belong 
to  a  different  embryological  category  from  the  axial  myotomic 
muscles. 

The  strongest  evidence  in  favor  of  the  myotomic  origin  of  the 
limb  muscles  is  that  furnished  by  their  nerve  supply,  this  presenting 
a  distinctly  segmental  arrangement.  This  does  not  necessarily 
imply,  however,  a  corresponding  primarily  metameric  arrangement 
of  the  muscles,  any  more  than  the  pronouncedly  segmental  arrange- 
ment of  the  cutaneous  nerves  implies  a  primary  metamerism  of  the 


THE    LIMB    MUSCLES 


211 


dermis  (see  p.  143).  It  may  mean  only  that  the  nerves,  being  seg- 
mental, retain  their  segmental  relations  to  one  another  even  in  their 
distribution  to  non-metameric  structures,  and  that,  consequently, 
the  limb  musculature  is  supplied  in  succession  from  one  border  of 
the  limb  bud  to  the  other  from  succeeding  nerve  roots. 

But  whether  further  observation  may  prove  or  disprove  the 
myotomic  origin  of  the  limb  musculature,  the  fact  remains  that  it 
possesses  a  segmentally  arranged  innervation,  and  it  is  possible, 


Fig.  123. — Diagram  of  a  Segment  of  the  Body  and  Limb. 
bl,  Axial  blastema;  dm,  dorsal  musculature  of  trunk;  rl,  nerve  to  limb;  s,  septum 
between  dorsal  and  ventral  trunk  musculature;  str.d,  dorsal  layer  of  limb  musculature; 
tr.d  and  tr.v,  dorsal  and  ventral  divisions  of  a  spinal  nerve;  vm,  ventral  musculature 
of  the  trunk. — (Kollmann.) 

therefore,  to  recognize  in  the  limb  buds  a  series  of  parallel  bands  of 
muscle  tissue,  extending  longitudinally  along  the  bud  and  each 
supplied  by  a  definite  segmental  nerve.  And  furthermore,  corre- 
sponding to  each  band  upon  the  ventral  (praeaxial)  surface  of  the 
limb  bud,  there  is  a  band  similarly  innervated  upon  the  dorsal  (post- 
axial)  surface,  since  the  fibers  which  pass  to  the  limb  from  each  nerve 
root  sooner  or  later  arrange  themselves  in  praeaxial  and  postaxial 


212 


THE    LIMB    MUSCLES 


groups  as  is  shown  in  the  diagram  Fig.  123.  The  first  nerve  which 
enters  the  limb  bud  lies  along  its  anterior  border,  and  consequently 
the  muscle  bands  which  are  supplied  by  it  will,  in  the  adult,  lie  along 


Fig.  124. — External  Surface  of  the  Os  Innominatum  showing  the  Attachment 

of  Muscles  and  the  Zones  Supplied  by  the  Various  Nerves. 

12,  Twelfth  thoracic  nerve;  I  to  V,  lumbar  nerves;  1  and  2,  sacral  nerves. — {Bolk.) 

the  outer  side  of  the  arm  and  along  the  inner  side  of  the  leg,  in  conse- 
quence of  the  rotation  in  opposite  directions  which  the  limbs  undergo 
during  development  (see  p.  101). 


THE    LIMB    MUSCLES 


213 


The  first  nerve  which  supplies  the  muscles  attached  to  the  dorsum 
of  the  ilium  is  the  second  lumbar,  and  following  that  there  come 
successively  from  before  backward  the  remaining  lumbar  and  the 


my 


Fig.  125. — Sections  through  (A)  the  Thigh  and  (B)  the  Calf  showing  the 
Zones  Supplied  by  the  Nerves.  The  Nerves  are  Numbered  in  Continuation 
with  the  Thoracic  Series. — (A,  after  Bolk.) 

first  and  second  sacral  nerves.  The  arrangement  of  the  muscle 
bands  supplied  by  these  nerves  and  the  muscles  of  the  adult  to  which 
they  contribute  may  be  seen  from  Fig.  124.  What  is  shown  there  is 
only  the  upper  portions  of  the  postaxial  bands,  their  lower  portions 


214 


THE    LIMB    MUSCLES 


extending  downward  on  the  anterior  surface  of  the  leg.  Only  the 
sacral  bands,  however,  extend  throughout  the  entire  length  of  the 
limb  into  the  foot,  the  second  lumbar  band  passing  down  only  to 
about  the  middle  of  the  thigh,  the  third  to  about  the  knee,  the  fourth 
to  about  the  middle  of  the  crus  and  the  fifth  as  far  as  the  base  of  the 
fifth  metatarsal  bone,  and  the  same  is  true  of  the  corresponding 
praeaxial  bands,  which  descend  from  the  ventral  surface  of  the  os 
coxae  (innominatum)  along  the  inner  and  posterior  surfaces  of  the 
leg  to  the  same  points.  The  first  and  second  sacral  bands  can  be 
traced  into  the  foot,  the  first  giving  rise  to  the  musculature  of  its 


Fig.  126. — Section  through  the  Upper  Part  of  the  Arm  showing  the  Zones 
Supplied  by  the  Nerves. 

$v  to  jv,  Ventral  branches;  5J  to  Sd,  dorsal  branches  of  the  cervical  nerves.— (Bolk.) 

inner  side  and  the  second  to  that  of  its  outer  side,  the  praeaxial  bands 
forming  the  plantar  musculature,  while  the  postaxial  ones  are  upon 
the  dorsum  of  the  foot  as  a  result  of  the  rotation  which  the  limb  has 
undergone. 

In  a  transverse  section  through  a  limb  at  any  level  all  the  muscle 
bands,  both  praeaxial  and  postaxial,  which  descend  to  that  level 
will  be  cut  and  will  lie  in  a  definite  succession  from  one  border  of  the 
limb  to  the  other,  as  is  seen  in  Fig.  125.  In  the  differentiation  of  the 
individual  muscles  which  proceeds  as  the  nerves  extend  from  the 
trunk  into  the  axial  mesenchyme  of  the  limb,  the  muscle  bands 


THE    LIMB    MUSCLES  215 

undergo  modifications  similar  to  those  already  described  as  occurring 
in  the  trunk  myotomes.  Thus,  each  of  the  muscles  represented  in 
Fig.  125,  B,  is  formed  by  the  fusion  of  elements  derived  from  two 
or  more  bands;  the  soleus  and  gastrocnemius  represent  deep  and 
superficial  layers  formed  from  the  same  bands  by  a  horizontal 
(tangential)  splitting,  these  same  muscles  contain  a  portion  of  the 
second  sacral  band  which  overlaps  muscles  composed  only  of  higher 
myotomes,  and  the  intermuscular  septum  between  the  peroneus 
brevis  and  the  flexor  hallucis  longus  represents  a  portion  of  the  third 
sacral  band  which  has  degenerated  into  connective  tissue. 

A  similar  arrangement  occurs  in  the  bands  which  are  to  be  recog- 
nized in  the  musculature  of  the  upper  limb.  These  are  supplied  by 
the  fourth,  fifth,  sixth,  seventh  and  eighth  cervical  and  the  first 
thoracic  nerves,  and  only  those  supplied  by  the  eighth  cervical  and 
the  first  thoracic  nerves  extend  as  far  as  the  tips  of  the  fingers.  The 
arrangement  of  the  bands  in  the  upper  part  of  the  brachium  may  be 
seen  from  Fig.  126,  in  connection  with  which  it  must  be  noted  that 
the  fourth  cervical  band  does  not  extend  down  to  the  level  at  which 
the  section  is  taken  and  that  the  praeaxial  band  of  the  eighth  cervical 
nerve  and  both  the  praeaxial  and  postaxial  bands  of  the  first  thoracic 
are  represented  only  by  connective  tissue  in  this  region. 

In  another  sense  than  the  longitudinal  one  there  is  a  division 
of  the  limb  musculature  into  more  or  less  definite  areas,  namely,  in  a 
transverse  direction  in  accordance  with  the  jointing  of  the  skeleton. 
Thus,  there  may  be  recognized  a  group  of  muscles  which  pass  from 
the  axial  skeleton  to  the  pectoral  girdle,  another  from  the  limb 
girdle  to  the  brachium  or  thigh,  another  from  the  brachium  or  thigh 
to  the  antibrachium  or  crus,  another  from  the  antibrachium  or  crus 
to  the  carpus  or  tarsus,  and  another  from  the  carpus  or  tarsus  to  the 
digits.  This  transverse  segmentation,  if  it  may  be  so  termed,  is  not, 
however,  perfectly  definite,  many  muscles,  even  in  the  lower  verte- 
brates, passing  over  more  than  one  joint,  and  in  the  mammalia, 
especially,  it  is  further  obscured  by  secondary  migrations,  by  the 
partial  degeneration  of  muscles  and  by  an  end  to  end  union  of 
primarily  distinct  muscles. 


2l6  THE    LIMB    MUSCLES 

The  latissimus  dorsi,  serratus  anterior  and  pectoral  muscles  are 
all  examples  of  a  process  of  migration  as  is  shown  by  their  innervation 
from  cervical  nerves,  as  well  as  by  the  actual  migration  which  has 
been  traced  in  the  developing  embryo  (Mall,  Lewis).  In  the  lower 
limb  evidences  of  migration  may  be  seen  in  the  femoral  head  of  the 
biceps,  comparative  anatomy  showing  this  to  be  a  derivative  of  the 
gluteal  set  of  muscles  which  has  secondarily  become  attached  to  the 
femur  and  has  associated  itself  with  a  praeaxial  muscle  to  form  a 
compound  structure.  An  appearance  of  migration  may  also  be 
produced  by  a  muscle  making  a  secondary  attachment  below  its 
original  origin  or  above  the  insertion  and  the  upper  or  lower  part, 
as  the  case  may  be,  then  degenerating  into  connective  tissue.  This 
has  been  the  case  with  the  peroneus  longus,  which,  in  the  lower 
mammals,  has  a  femoral  origin,  but  has  in  man  a  new  origin  from 
the  fibula,  its  upper  portion  being  represented  by  the  fibular  lateral 
ligament  of  the  knee-joint.  So  too  the  pectoralis  minor  is  primarily 
inserted  into  the  humerus,  but  it  has  made  a  secondary  attachment 
to  the  coracoid  process,  its  distal  portion  forming  a  coraco-humeral 
ligament. 

The  comparative  study  of  the  flexor  muscles  of  the  antibrachial 
and  crural  regions  has  yielded  abundant  evidence  of  extensive 
modifications  in  the  differentiation  of  the  limb  muscles.  In  the 
tailed  amphibia  these  muscles  are  represented  by  a  series  of  super- 
posed layers,  the  most  superficial  of  which  arises  from  the  humerus 
or  femur,  while  the  remaining  ones  take  their  origin  from  the  ulna 
or  fibula  and  are  directed  distally  and  laterally  to  be  inserted  either 
into  the  palmar  or  plantar  aponeurosis,  or,  in  the  case  of  the  deeper 
layers,  into  the  radius  (tibia)  or  carpus  (tarsus).  In  the  arm  of  the 
lower  mammalia  the  deepest  layer  becomes  the  pronator  quadratus, 
the  lateral  portions  of  the  superficial  layer  are  the  flexor  carpi  ulnaris 
and  the  flexor  carpi  radialis,  while  the  intervening  layers,  together 
with  the  median  portion  of  the  superficial  one,  assuming  a  more 
directly  longitudinal  direction,  fuse  to  form  a  common  flexor  mass 
which  acts  on  the  digits  through  the  palmar  aponeurosis.  From 
this  latter  structure  and  from  the  carpal  and  metacarpal  bones  five 


THE    LIMB    MUSCLES 


217 


layers  of  palmar  muscles  take  origin.  The  radial  and  ulnar  portions 
of  the  most  superficial  of  these  become  the  flexor  pollicis  brevis  and 
abductor  pollicis  brevis  and  the  abductor  quinti  digiti,  while  the  rest 
of  the  layer  degenerates  into  connective  tissue,  forming  tendons 


Fig.  127. — Transverse  sections  through  (A)  the  forearm  and  (B)  the  hand  showing 
the  arrangement  of  the  layers  of  the  flexor  muscles.  The  superficial  layer  is  shaded 
horizontally,  the  second  layer  vertically,  the  third  obliquely  to  the  left,  the  fourth 
vertically,  and  the  fifth  obliquely  to  the  right.  AbM,  abductor  digiti  quinti;  AdP, 
adductor  pollicis;  BR,  brachio-radialis;  ECD,  extensor  digitorum  communis;  ECU, 
extensor  carpi  ulnaris;£Z,  extensor  indicis;  EMD,  extensor  digiti  quinti;  EMP,  abductor 
pollicis  longus;  ERB,  extensor  carpi  radialis  brevis;  FCR,  flexor  carpi  radialis;  FCU, 
flexor  carpi  ulnaris;  FLP,  flexor  pollicis  longus;  FM,  flexor  digiti  quinti  brevis;  FP, 
flexor  digitorum  profundus;  FS,  flexor  digitorum  sublimis;  ID,  interossei  dorsales; 
IV,  interossei  volares;  L,  lumbricales;  OM,  opponens  digiti  quinti;  PL,  palmaris 
longus;  PT,  pronator  teres;  R,  radius;  U,  ulna;  II-V,  second  to  fifth  metacarpal. 

which  pass  to  the  four  ulnar  digits.  Gradually  superficial  portions 
of  the  antibrachial  flexor  mass  separate  off,  carrying  with  them  the 
layers  of  the  palmar  aponeurosis  from  which  the  tendons  representing 


2l8 


THE    LIMB    MUSCLES 


the  superficial  layer  of  the  palmar  muscles  arise,  and  they  form  with 
these  the  flexor  digitorum  sublimis.  The  deeper  layers  of  the  anti- 
brachial  flexor  mass  become  the  flexor  digitorum  profundus  and 
the  flexor  pollicis  longus  (Fig.  127,  A),  and  retain  their  connection 
with  the  deeper  layers  of  the  palmar  aponeurosis  which  form 
their  tendons;  and  since  the  second  layer  of  the  palmar  muscles 
takes  origin  from  this  portion  of  the  aponeurosis  it  becomes  the 
lumbrical  muscles,  arising  from  the  profundus  tendons  (Fig.  127, 


Fig.  128. — Transverse  sections  through  (A)  the  crus  and  (B)  the  foot,  showing  the 
arrangement  of  the  layers  of  the  flexor  muscles.  The  shading  has  the  same  significance 
as  in  the  preceding  figure.  AbH,  abductor  hallucis;  AbM,  abductor  minimi  digiti; 
AdH,  adductor  hallucis;  ELD,  extensor  longus  digitorum;  F,  fibula;  FBD,  flexor 
brevis  digitorium;  FBH,  flexor  brevis  hallucis;  FBM,  flexor  brevis  minimi  digiti; 
FLD,  flexor  longus  digitorum;  G,  gastrocnemius;  ID,  interossei  dorsalis;  IV,  interossei 
ventrales;  L,  lumbricales;  P,  plantaris;  Pe,  peroneus  longus;  Po,  popliteus;  S,  soleus; 
T,  tibia;  TA,  tibialis  anticus;  TP,  tibialis  posticus;  I-V,  first  to  fifth  metatarsal. 

B).  The  third  layer  of  palmar  muscles  becomes  the  adductors 
of  the  digits,  reduced  in  man  to  the  adductor  pollicis,  while  from 
the  two  deepest  layers  the  interossei  are  developed.  Of  these 
the  fourth  layer  consists  primarily  of  a  pair  of  slips  correspond- 
ing to  each  digit,  while  the  fifth  is  represented  by  a  series  of  muscles 
which  extend  obliquely  across  between  adjacent  metacarpals. 
With  these  last  muscles  certain  of  the  fourth  layer  slips  unite  to  form 
the  dorsal  interossei,  while  the  rest  become  the  volar  interossei. 
j  The  modifications  of  the  almost  identical  primary  arrangement 
in  the  crus  and  foot  are  somewhat  different.     The  superficial  layer 


LITERATURE  210, 

of  the  crural  flexors  becomes  the  gastrocnemius  and  plantaris  (Fig. 
128,  A)  and  the  deepest  layer  becomes  the  popliteus  and  the  inter- 
osseous membrane.  The  second  and  third  layers  unite  to  form  a 
common  mass  which  is  inserted  into  the  deeper  layers  of  the  plantar 
aponeurosis  and  later  differentiates  into  the  soleus  and  the  long 
digital  flexor,  the  former  shifting  its  insertion  from  the  plantar 
aponeurosis  to  the  os  calcis,  while  the  flexor  retains  its  connection 
with  the  deeper  layers  of  the  aponeurosis,  these  separating  from  the 
superficial  layer  to  form  the  long  flexor  tendons.  The  fourth  layer 
partly  assumes  a  longitudinal  direction  and  becomes  the  tibialis 
posterior  and  the  flexor  hallucis  longus  and  partly  retains  its  original 
cblique  direction  and  its  connection  with  the  deep  layers  of  the 
plantar  aponeurosis,  becoming  the  quadratus  plantse.  In  the  foot 
(Fig.  128,  B)  the  superficial  layer  persists  as  muscular  tissue,  forming 
the  abductors,  the  flexor  digitorum  brevis  and  the  medial  head  of  the 
flexor  hallucis  brevis,  the  second  layer  becomes  the  lumbricales,  and 
the  third  the  lateral  head  of  the  flexor  hallucis  brevis  and  the  adduc- 
tor hallucis,  while  the  fourth  and  fifth  layers  together  form  the  ioter- 
ossei,  as  in  the  hand,  the  flexor  quinti  digiti  brevis  really  belonging 
to  that  group  of  muscles. 

LITERATURE. 

C.  R.  Bardeen  and  W.  H.  Lewis:  "Development  of  the  Limbs,  Body-wall,  and 

Back  in  Man,"     The  American  Journal  of  Anat.,  1,  1901. 
K.  Bardeleben:  "Musk el    und   Fascia,"    Jenaische    Zeitschr.  fiir   Naturwissensch., 

xv,  1882. 
L.  Bolk:  "Beziehungen  zwischen  Skelett,  Muskulatur  und  Nerven  der  Extremitaten, 

dargelegt   am   Beckengurtel,   an   dessen   Muskulatur   sowie   am  Plexus   lumbo- 

sacralis,"  Morphol.  Jahrbuch,  xxi,  1894. 
L.  Bolk:  "  Rekonstruktion  der  Segmentirung  der  Gliedmassenmuskulatur  dargelegt 

an  den  Muskeln  des  Oberschenkels  und  des  Schultergurtels,"  Morphol.  Jahrbuch, 

xxii,  1895. 
L.  Bolk:  "Die  Sklerozonie  des  Humerus,"  Morphol.  Jahrbuch,  xxill,  1S96. 
L.  Bolk:  "Die    Segmentaldifferenzierung    des    menschlichen    Rumpfes    und    seiner 

Extremitaten,"  1,  Morphol.  Jahrbuch,  xxv,  1898. 
R.  Futamtjra:  "Ueber   die  Entwickelung   der  Facialismuskulatur   des  Menschen," 

Anat.  Hefte,  xxx,  1906. 
E.  Godlewski:  "Die  Entwicklung  des  Skelet-  und  Herzmuskelgewebes  der  Sauge- 

thiere,"  Archiv  fur  mikr.  Anat.,  lx,  1902. 


220  LITERATURE 

E.  Grafenberg:  "Die  Entwicklung  der  menschlichen  Beckenmuskulatur,"   Anal. 

Hefte,  xxiii,  1904. 
W.  P.  Herringham:  "The  Minute  Anatomy  of  the  Brachial  Plexus,"  Proceedings 

of  the  Royal  Soc.  London,  xli,  1886. 
W.  H.  Lewis:  "  The  Development  of  the  Arm  in  Man,"  Amer.  Jour,  of  Anat.,  1,  1902 
J.  B.  MacCallum:  "On  the  Histology  and  Histogenesis  of  the  Heart  Muscle-cell," 

Anat.  Anzeiger,  xiil,  1897. 
J.  B.  MacCallum:  "On    the    Histogenesis    of    the    Striated    Muscle-fiber   and  the 

Growth  of  the  Human  Sartorius  Muscle,"  Johns  Hopkins  Hospital  Bulletin,  1898 

F.  P.  Mall:  "Development  of  the  Ventral  Abdominal  Walls  in  Man,"  Journ.  of 

Morphol.,  xiv,  1898. 
Caroline  McGill:  "The  Histogenesis  of  Smooth  Muscle  in  the  Alimentary  Canal 

and  Respiratory  Tract  of  the  Pig,"  Internat.  Monatschr.  Anat.  und  Phys.,  xxiv, 

1907. 
J.  P.  McMurrich:  "The  Phylogeny  of  the  Forearm  Flexors,"  Amer.  Journ,  of  Anat., 

11,  1903. 
J.  P.  McMurrich:  "The  Phylogeny  of  the  Palmar  Musculature,"  Amer.  Journ.  of 

Anat.,  11,  1903. 
J.  P.  McMurrich:  "The  Phylogeny  of  the  Crural  Flexors,"  Amer.  Journ.  of  Anat., 

iv,  1904. 
J.  P.  McMurrich:  "The  Phylogeny  of  the  Plantar  Musculature,"  Amer.  Journ.  of 

Anat.,  vi,  1907. 

A.  Meek:  "Preliminary  Note  on  the  Post-embryonal  History  of  Striped  Muscle-fibers 

in  Mammalia,"  Anat.  Anzeiger,  xiv,  1898.     (See  also  Anat.  Anzeiger,  xv,  1899.) 

B.  Morpurgo:  "Ueber  die  post-embryonale  Entwickelung  der  quergestreiften  Muskel 

von  weissen  Ratten,"  Anat.  Anzeiger,  xv,  1899. 
I.  Popowsky:  "  Zur  Entwicklungsgeschichte  des  N.  facialis  beim  Menschen,"  Morphol. 

Jahrbuch,  xxiii,  1896. 
I.  Popowsky:  "  Zur  Entwickelungsgeschichte  der  Dammmuskulatur  beim  Menschen," 

Anat.  Hefte,  xi,  1899. 
L.  Rethi:  "Der  peripheren  Verlauf  der  motorischen  Rachen-  und  Gaumennerven," 

Sitzungsber.  der  kais.  Akad.  Wissensch.  Wien.  Math.-Naturwiss.  Classe,  Cii,  1893. 

C.  S.  Sherrington:  "  Notes  on  the  Arrangement  of  Some  Motor  Fibers  in  the  Lumbo- 

sacral Plexus,"  Journal  of  Physiol.,  xin,  1892. 
J.  B.  Sutton:  "Ligaments,  their  Nature  and  Morphology,"  London,  1897. 


CHAPTER  IX. 

THE  DEVELOPMENT  OF  THE  CIRCULATORY  AND  LYM- 
PHATIC SYSTEMS. 

At  present  nothing  is  known  as  to  the  earliest  stages  of  develop- 
ment of  the  circulatory  system  in  the  human  embryo,  but  it  may  be 
supposed  that  they  resemble  in  their  fundamental  features  what  has 
been  observed  in  such  forms  as  the  rabbit  and  the  chick.  In  both 
these  the  system  originates  in  two  separate  parts,  one  of  which, 
located  in  the  embryonic  mesoderm,  gives  rise  to  the  heart,  while  the 
other,  arising  in  the  extra-embryonic  mesoderm,  forms  the  first 
blood-vessels.  It  will  be  convenient  to  consider  these  two  parts 
separately,  and  the  formation  of  the  blood-vessels  may  be  first 
described. 

In  the  rabbit  the  extension  of  the  mesoderm  from  the  embryonic 
region,  where  it  first  appears,  over  the  yolk-sac  is  a  gradual  process, 
and  it  is  in  the  more  peripheral  portions  of  the  layer  that  the  blood- 
vessels first  make  their  appearance.  They  can  be  distinguished 
before  the  splitting  of  the  mesoderm  has  been  completed,  but  are 
always  developed  in  that  portion  of  the  layer  which  is  most  intimately 
associated  with  the  yolk-sac,  and  consequently  becomes  the  splanch- 
nic layer.  They  belong,  indeed,  to  the  deeper  portion  of  that  layer, 
that  nearest  the  endoderm  of  the  yolk-sac,  and  so  characteristic  is 
their  origin  from  this  portion  of  the  layer  that  it  has  been  termed  the 
angioblast  and  has  been  held  to  be  derived  from  the  endoderm 
independently  of  the  mesoderm  proper.  The  first  indication  of 
blood-vessels  is  the  appearance  in  the  peripheral  portion  of  the 
mesoderm  of  cords  or  minute  patches  of  spherical  cells  (Fig.  129,  .4). 
These  increase  in  size  by  the  division  and  separation  of  the  cells  from 
one  another  (Fig.  129,  B),  a  clear  fluid  appearing  in  the  intervals 
which  separate  them.     Soon  the  cells  surrounding  each  cord  arrange 


222 


DEVELOPMENT    OF    THE    BLOOD-VESSELS 


themselves  to  form  an  enclosing  wall,  and  the  cords,  increasing  in 
size,  unite  together  to  form  a  network  of  vessels  in  which  float  the 
spherical  cells  which  may  be  known  as  mesamceboids  (Minot). 
Viewed  from  the  surface  at  this  stage  a  portion  of  the  vascular  area 
of  the  mesoderm  would  have  the  appearance  shown  in  Fig.  130, 
revealing  a  dense  network  of  canals  in  which,  at  intervals,  are 
groups  of  mesamaeboids  adherent  to  the  walls,  constituting  what  have 
been  termed  the  blood-islands,  while  in  the  meshes  of  the  network 
unaltered  mesoderm  cells  can  be  seen,  forming  the  so-called  sub- 
stance-islands. 


Fig.  129. — Transverse  Section  through  the  Area  Vasculosa  of  Rabbit 
Embryos  showing  the  Transformation  of  Mesoderm  cells  into  the  Vascular 
Cords. 

Ec,  Ectoderm;  En,  endoderm;  Me,  mesoderm. — {van  der  Stricht.) 

At  the  periphery  of  the  vascular  area  the  vessels  arrange  them- 
selves to  form  a  sinus  terminalis  enclosing  the  entire  area,  and  the 
vascularization  of  the  splanchnic  mesoderm  gradually  extends 
toward  the  embryo.  Reaching  it,  the  vessels  penetrate  the  embry- 
onic tissues  and  eventually  come  into  connection  with  the  heart, 
which  has  already  differentiated  and  has  begun  to  beat  before  the 
connection  with  the  vessels  is  made,  so  that  when  it  is  made  the 
circulation  is  at  once  established.  Before,  however,  the  vasculariza- 
tion reaches  the  embryo  some  of  the  canals  begin  to  enlarge  (Fig. 


DEVELOPMENT    OF    THE    BLOOD-VESSELS 


223 


B£ 


131,-4),  producing  arteries  and  veins,  the  rest  of  the  network  forming 
capillaries  uniting  these  two  sets  of  vessels,  and,  this  process  continu- 
ing, there  are  eventually  differentiated  a  single  vitelline  artery  and 
two  vitelline  veins  (Fig.  131,  B). 

In  the  human  embryo  the  small  size  of  the  yolk-sac  permits  of  the 
extension  of  the  vascular  area  over 
its  entire  surface  at  an  early  period, 
and  this  condition  has  already  been 
reached  in  the  earliest  stages  known 
and  consequently  no  sinus  termin- 
alis  such  as  occurs  in  the  rabbit  is 
visible.  Otherwise  the  conditions 
are  probably  similar  to  what  has 
been  described  above,  the  first  cir- 
culation developed  being  associated 
with  the  yolk-sac. 

It  is  to  be  noted  that  the  capil- 
lary network  of  the  area  vasculosa 
consists  of  relatively  wide  anasto- 
mosing spaces  whose  endothelial 
lining  rests  directly  upon  the  sub- 
stance islands  (Fig.  130).  In  cer- 
tain of  the  embryonic  organs,  not- 
ably the  liver,  the  metanephros 
and  the  heart,  the  network  has  a 
similar  character,  consisting  of  wide 
anastomosing  spaces  bounded  by 
an  endothelium  which  rests  di- 
rectly, or  almost  so,  upon  the  par- 
enchyma of  the  organ  (the  hepatic 
cylinders,  the  mesonephric  tubules,  or  the  cardiac  muscle  trabecular) 
(Figs.  132  and  190,  B).  To  this  form  of  capillary  the  term  sinusoid 
has  been  applied  (Minot),  and  it  appears  to  be  formed  by  the  expan- 
sion of  the  wall  of  a  previously  existing  blood-vessel,  which  thus 
moulds  itself,  as  it  were,  over  the  parenchyma  of  the  organ.     The 


Fig.  130. — Surface  View  of  a 
Portion  of  the  Area  Vasculosa  of 
a  Chick. 

The  vascular  network  is  represented 
by  the  shaded  portion.  Bi,  Blood- 
island;  Si,    substance-island. — (Disse.) 


224 


THE    FORMATION    OF    THE    BLOOD 


true  capillaries,  on  the  other  hand,  are  more  definitely  tubular  in 
form,  are  usually  imbedded  in  mesenchymatous  connective  tissue 
and  are  developed  in  the  same  manner  as  the  primary  capillaries 
of  the  area  vasculosa,  by  the  aggregation  of  vasifactive  cells  to  form 
cords,  and  the  subsequent  hollowing  out  of  these.  Whether  these 
vasifactive  cells  are  new  differentiations  of  the  embryonic  mesen- 
chyme or  are  budded  off  from  the  walls  of  existing  capillaries  which 
have  grown  in  from  extra-embryonic  regions,  is  at  present  undecided. 
The  Formation  of  the  Blood. — The  mesamceboids,  which  are 


gl 


i  i 


A  , \ 

Fig.  131. — The  Vascular  Areas  of  Rabbit  Embryos.  In  B  the  Veins  are 
Represented  by  Black  and  the  Network  is  Omitted. — (van  Beneden  and 
Julin.) 


the  first  formed  blood-corpuscles  are  all  nucleated  and  destitute  or 
nearly  so  of  haemoglobin.  They  have  been  held  by  some  observers 
to  be  the  only  source  of  the  various  forms  of  corpuscles  that  are 
found  in  the  adult  vessels,  while  others  maintain  that  they  give  rise 
only  to  the  red  corpuscles,  the  leukocytes  arising  in  tissues  external 
to  the  blood-vessels  and  only  secondarily  making  their  way  into 
them.  According  to  this  latter  view  the  red  and  white  corpuscles 
have  a  different  origin  and  remain  distinct  throughout  life. 


THE   FORMATION   OF   THE   BLOOD  225 

So  long  as  the  formation  of  blood-vessels  is  taking  place  in  the 
extra-embryonic  mesoderm,  so  long  are  new  mesamceboids  being 
differentiated  from  the  mesoderm.  But  whether  the  formation  of 
blood-vessels  within  the  embryo  results  from  a  differentiation  of  the 
embryonic  mesoderm  in  situ,  or  from  the  actual  ingrowth  of  vessels 
from  the  extra-embryonic  regions  (His),  is  as  yet  uncertain,  and 
hence  it  is  also  uncertain  whether  mesamceboids  are  differentiated 
from  the  embryonic  mesoderm  or  merely  pass  into  the  embryonic 
region  from  the  more  peripheral  areas.  However  this  may  be,  it 
is  certain  that  they  and  the  erythrocytes  that  are  formed  from  them 
increase  by  division  in  the  interior  of  the  embryo,  and  that  there 
are  certain  portions  of  the  body  in  which  these  divisions  take  place 
most  abundantly,  partly,  perhaps,  on  account  of  the  more  favorable 
conditions  of  nutrition  which  they  present  and  partly  because  they  are 
regions  where  the  circulation  is  sluggish  and  permits  the  accumula- 
tion of  erythrocytes.  These  regions  constitute  what  have  been 
termed  the  hematopoietic  organs,  and  are  especially  noticeable  in  the 
later  stages  of  fetal  life,  diminishing  in  number  and  variety  about  the 
time  of  birth.  It  must  be  remembered,  however,  that  the  life  of 
individual  corpuscles  is  comparatively  short,  their  death  and  dis- 
integration taking  place  continually  during  the  entire  life  of  the 
individual,  so  that  there  is  a  necessity  for  the  formation  of  new 
corpuscles  and  for  the  existence  of  haematopoietic  organs  at  all 
stages  of  life. 

In  the  fetus  mesamceboids  in  process  of  division  may  be  found  in 
the  general  circulation  and  even  in  the  heart  itself,  but  they  are  much 
more  plentiful  in  places  where  the  blood-pressure  is  diminished,  as, 
for  instance,  in  the  larger  capillaries  of  the  lower  limbs  and  in  the 
capillaries  of  all  the  visceral  organs  and  of  the  subcutaneous  tissues. 
Certain  organs,  however,  such  as  the  liver,  the  spleen,  and  the 
bone-marrow,  present  especially  favorable  conditions  for  the  multi- 
plication of  the  blood-cells,  and  in  these  not  only  are  the  capillaries 
enlarged  so  as  to  afford  resting-places  for  the  corpuscles,  but  gaps 
appear  in  the  walls  of  the  vessels  through  which  the  blood-elements 
may  pass  and  so  come  into  intimate  relations  with  the  actual  tissues 
is 


226 


THE  FORMATION  OF  THE  BLOOD 


of  the  organs  (Fig.  132).  After  birth  the  haematopoietic  function  of 
the  liver  ceases  and  that  of  the  spleen  becomes  limited  to  the  forma- 
tion of  white  corpuscles,  though  the  complete  function  may  be 
re-established  in  cases  of  extreme  anaemia.  The  bone-marrow, 
however,  retains  the  function  completely,  being  throughout  life  the 
seat  of  formation  of  both  red  and  white  corpuscles,  the  lymphatic 
nodes  and  follicles,  as  well  as  the  spleen,  assisting  in  the  formation 
of  the  latter  elements. 

The  mesamceboids  early  become  converted  into  nucleated  red 

corpuscles  or  erythrocytes  by 
the  development  of  haemoglo- 
bin in  their  cytoplasm,  their 
nuclei  at  the  same  time  be- 
coming granular.  Up  to  a 
stage  at  which  the  embryo  has 
a  length  of  about  12  mm.  these 
are  the  only  form  of  red  cor- 
puscle in  the  circulation,  but 
at  this  time  (Minot)  a  new 
form,  characterized  by  its 
smaller  size  and  more  deeply 
staining  nucleus,  makes  its  ap- 
pearance. These  erythrocytes 
have  been  termed  normoblasts 
(Ehrlich),  although  they  are 
merely  transition  stages  lead- 
ing to  the  formation  of  erythro- 
plastids  by  the  extrusion  of  their  nuclei  (Fig.  133).  The  cast-off 
nuclei  undergo  degeneration  and  phagocytic  absorption  by  the 
leukocytes,  and  the  masses  of  cytoplasm  pass  into  the  circulation, 
becoming  more  and  more  numerous  as  development  proceeds, 
until  finally  they  are  the  typical  haemoglobin-containing  elements 
in  the  blood  and  form  what  are  properly  termed  the  red  blood- 
corpuscles. 

It  has  already  (p.  224)  been  pointed  out  that  discrepant  views 


Fig.  132. — Section  of  a  Portion  or 
the  Liver  of  a  Rabbit  Embryo  of  5  mm. 
e,  Erythrocytes  in  the  liver  substance  and 
in  a  capillary;  h,  hepatic  cells. — {van  der 
Stricht) 


THE    FORMATION    OF    THE    BLOOD  227 

prevail  as  to  the  origin  of  the  white  blood-corpuscles.  Indeed,  three 
distinct  modes  of  origin  have  been  assigned  to  them.  According  to 
one  view  they  have  a  common  origin  with  the  erythrocytes  from  the 
mesamceboids  (Weidenreich),  according  to  another  they  are  formed 
from  mesenchyme  cells  outside  the  cavities  of  the  blood-vessels 
(Maximo w),  while  according  to  a  third  view  the  first  formed  leuko- 
cytes take  their  origin  from  the  endodermal  epithelial  cells  of  the 
thymus  gland  (Prenant). 

But  whatever  may  be  their  origin  in  later  stages  the  leukocytes 
multiply  by  mitosis  and  there  is  a  tendency  for  the  dividing  cells  to 
collect  in  the  lymphoid  tissues,  such  as 

the  lymph  nodes,  tonsils,  etc.,  to  form  /||\  /^s    0$\   /^p.       & 
more  or   less  definite   groups  which     —    ^^     v^    KjJ      \Jj 

have  been  termed  germ-centers  (Flem- 

.        m,                   ..       .          .        _  Fig.    133. — Stages    in    the 

ming).     The  new  cells  when  they  first  transformation   of   an   Ery- 

pass  into  the  circulation  have  a  rel-  throcyte    into    an  _  Erythro- 

r  plastid. — (van  der  Stricnt.) 

atively  large  nucleus  surrounded  by  a 

small  amount  of  cytoplasm  without  granules  and,  since  they  resemble 
the  cells  found  in  the  lymphatic  vessels,  are  termed  lymphocytes 
(Fig.  134,  a).  In  the  circulation,  however,  other  forms  of  leukocytes 
also  occur,  which  are  believed  to  have  their  origin  from  cells  with 
much  larger  nuclei  and  more  abundant  cytoplasm,  which  occur 
throughout  life  in  the  bone-marrow  and  have  been  termed  myelo- 
cytes. Cells  of  a  similar  type,  free  in  the  circulation,  constitute 
what  are  termed  the  finely  granular  leukocytes  (neutrophile  cells  of 
Ehrlich)  (Fig.  134,  b),  but  whether  these  and  the  myelocytes  are 
derived  from  lymphocytes  or  have  an  independent  origin  is  as  yet 
undetermined.  Less  abundant  are  the  coarsely  granular  leukocytes 
(eosinophile  cells  of  Ehrlich)  (Fig.  134,  c),  characterized  by  the  coarse- 
ness and  staining  reactions  of  their  cytoplasmic  granules  and  by 
their  reniform  or  constricted  nucleus.  They  are  probably  deriva- 
tives of  the  finely  granular  type  and  it  has  been  maintained  by 
Weidenreich  that  their  granules  have  been  acquired  by  the  phago- 
cytosis of  degenerated  erythrocytes.  Finally,  a  third  type  is  formed 
by  the  polymorphonuclear  or  polynuclear  leukocytes  (basophile  cells 


228 


THE  FORMATION  OF  THE  BLOOD 


of  Ehrlich)  (Fig.  134,  d),  which  are  to  be  regarded  as  leukocytes  in 
the  process  of  degeneration  and  are  characterized  by  their  irregu- 
larly lobed  or  fragmented  nuclei,  as  well  as  by  their  staining 
peculiarities. 

In  the  fetal  haematopoietic  organs  and  in  the  bone-marrow  of  the 
adult  large,  so-called  giant-cells  are  found,  which,  although  they  do 
not  enter  into  the  general  circulation,  are  yet  associated  with  the 
development  of  the  blood-corpuscles.     These  giant-cells  as  they 


Fig.  134. — Figures  of  the  Different  Forms  of  White  Corpuscles  occurring 

in  Human  Blood. 

a,  Lymphocytes;  b,  finely  granular  (neutrophile)  leukocyte;  c,  coarsely  granular  (eosino- 

phile)  leukocyte;  d,  polymorphonuclear  (basophile)  leukocyte. — (Weidenreich.) 


occur  in  the  bone-marrow  are  of  two  kinds  which  seem  to  be  quite 
distinct,  although  both  are  probably  formed  from  leukocytes.  In 
one  kind  the  cytoplasm  contains  several  nuclei,  wherce  they  have 
been  termed  polycaryocytes,  and  they  seem  to  be  the  cells  which  have 
already  been  mentioned  as  osteoclasts  (p.  158).  In  the  other  kind 
(Fig-  I35)  tne  nucleus  is  single,  but  it  is  large  and  irregular  in  shape, 
frequently  appearing  as  if  it  were  producing  buds.  These  mega- 
caryocytes  appear  to  be  phagocytic  cells,  having  as  their  function  the 
destruction  of  degenerated  corpuscles  and  of  the  nuclei  of  the 
erythrocytes. 


THE    FORMATION    OF    THE   HEART  229 

The  blood-platelets  have  recently  been  shown  by  Wright  to  be 
formed  from  the  cytoplasm  of  the  megacaryocytes,  by  the  constric- 
tion and  separation  of  portions  of  the  slender  processes  to  which 
they  give  rise  in  their  amoeboid  movements  (Fig.  135). 


Fig.  135. — Megacaryocyte  from  a  Kitten,  which  has  Extended  two 
pseudopodial  processes  through  the  wall  of  blood-vessel  and  is  budding 
off  blood-platelets. 

bp,  Blood-platelets;  V,  blood-vessel. — (J.  H.  Wright.) 

The  Formation  of  the  Heart. — The  heart  makes  its  appearance 
while  the  embryo  is  still  spread  out  upon  the  surface  of  the  yolk-sac, 
and  arises  as  two  separate  portions  which  only  later  come  into  con- 
tact in  the  median  line.  On  each  side  of  the  body  near  the  margins 
of  the  embryonic  area  a  fold  of  the  splanchnopleure  appears,  pro- 
jecting into  the  ccelomic  cavity,  and  within  this  fold  a  very  thin- 
walled  sac  is  formed,  probably  by  a  splitting  off  of  its  innermost 
cells  (Fig.  136,  .4).  Each  fold  will  produce  a  portion  of  the  muscular 
walls  {myocardium)  of  the  heart,  and  each  sac  part  of  its  endothelium 
{endocardium).  As  the  constriction  of  the  embryo  from  the  yolk-sac 
proceeds,  the  two  folds  are  gradually  brought  nearer  together  (Fig. 
136,  B),  until  they  meet  in  the  mid-ventral  line,  when  the  myocardial 
folds  and  endocardial  sacs  fuse  together  (Fig.  136,  C)  to  form  a 
cylindrical  heart  lying  in  the  mid-ventral  line  of  the  body,  in  front 
of  the  anterior  surface  of  the  yolk-sac  and  in  what  will  later  be  the 


230 


THE    FORMATION    OF    THE    HEART 


cervical  region  of  the  body.     At  an  early  stage  the  various  veins 
which  have  already  been  formed,  the  vitellines,  umbilicals,  jugulars 


en 


Fig.    136. — Diagrams    Illustrating    the   Formation    or    the    Heart    in    the 

Guinea-pig. 
The  mesoderm  is  represented  in  black  and  the  endocardium  by  a  broken  line. 
am,    Amnion;    en,    endoderm;    h,    heart;    i,    digestive    tract.- — {After    Strahl    and 
Carius.) 

and  cardinals,  unite  together  to  open  into  a  sac-like  structure,  the 
sinus  venosus,  and  this  opens  into  the  posterior  end  of  the  heart 
cylinder.     The  anterior  end  of  the  cylinder  tapers  off  to  form  the 


THE    FORMATION    OF    THE   HEART 


231 


aortic  bulb,  which  is  continued  forward  on  the  ventral  surface  of  the 
pharyngeal  region  and  carries  the  blood  away  from  the  heart.  The 
blood  accordingly  opens  into  the  posterior  end  of  the  heart  tube  and 
flows  out  from  its  anterior  end. 

The  simple  cylindrical  form  soon  changes,  however,  the  heart 
tube  in  embryos  of  2.15  mm.  in  length  having  become  bent  upon 
itself  into  a  somewhat  S-shaped  curve  (Fig.  137).  Dorsally  and  to 
the  left  is  the  end  into  which  the  sinus  venosus  opens,  and  from  this 


Fig.  137. — Heart  of  EmbrycTof 
2.15  mm.,  from  a  Reconstruction. 

a,  Atrium;  ab,  aortic  bulb;  d,  dia- 
phragm; dc,  ductus  Cuvieri;  /,  liver; 
v,  ventricle;  vj,  jugular  vein;  vu,  um- 
bilical vein. — (His.) 


Fig.  138. — Heart  of  Embryo  of 
4.2  mm.,  seen  from  the  Dorsal 
Surface. 

DC,  Ductus  Cuvieri;  I A ,  left  atrium 
rA,  right  atrium;  vf,  jugular  vein;  VI, 
left  ventricle;  vu,  umbilical  vein. — 
{His.) 


the  heart  tube  ascends  somewhat  and  then  bends  so  as  to  pass  at 
first  ventrally  and  then  caudally  and  to  the  right,  where  it  again 
bends  at  first  dorsally  and  then  anteriorly  to  pass  over  into  the  aortic 
bulb.  The  portion  of  the  curve  which  lies  dorsally  and  to  the  left 
is  destined  to  give  rise  to  both  atria,  the  portion  which  passes  from 
right  to  left  represents  the  future  left  ventricle,  while  the  succeeding 
portion  represents  the  right  ventricle.  In  later  stages  (Fig.  138) 
the  left  ventricular  portion  drops  downward  in  front  of  the  atrial 


232 


THE    FORMATION    OF    THE   HEART 


portion,  assuming  a  more  horizontal  position,  while  the  portion 
which  represents  the  right  ventricle  is  drawn  forward  so  as  to  lie  in 
the  same  plane  as  the  left. 

At  the  same  time  two  small  out-pouchings  develop  from  the 
atrial  part  of  the  heart  and  form  the  first  indications  of  the  two 
atria.  As  development  progresses,  these  increase  in  size  to  form 
large  pouches  opening  into  a  common  atrial  canal  (Fig.  139)  which 
is  directly  continuous  with  the  left  ventricle,  and  as  the  enlarge- 
ment of  the  pouches  continues  their  openings  into  the  canal  enlarge, 

until  finally  the  pouches  become 
continuous  with  one  another, 
forming  a  single  large  sac,  and 
the  atrial  canal  becomes  reduced 
to  a  short  tube  which  is  slightly 
invaginated  into  the  ventricle 
(Fig.  140). 

In  the  meantime  the  sinus 
venosus,  which  was  originally  an 
oval  sac  and  opened  into  the 
atrial  canal,  has  elongated  trans- 
versely until  it  has  assumed  the 
form  of  a  crescent  whose  convex- 
ity is  in  contact  with  the  walls  of 
the  atria,  and  its  opening  into  the 
heart  has  verged  toward  the  right,  until  it  is  situated  entirely  within  the 
area  of  the  right  atrium.  As  the  enlargement  of  the  atria  continues, 
the  right  horn  and  median  portion  of  the  crescent  are  gradually  taken 
up  into  their  walls,  so  that  the  various  veins  which  originally  opened 
into  the  sinus  now  open  directly  into  the  right  atrium  by  a  single 
opening,  guarded  by  a  projecting  fold  which  is  continued  upon  the 
roof  of  the  atrium  as  a  muscular  ridge  known  as  the  septum  spurium 
(Fig.  140,  sp).  The  left  horn  of  the  crescent  is  not  taken  up  into 
the  atrial  wall,  but  remains  upon  its  posterior  surface  as  an  elongated 
sac  forming  the  coronary  sinus. 

The  division  of  the  now  practically  single  atrial  cavity  into  the 


Fig.  139. — Heart  of  Embryo  of  5 
mm.,  Seen  from  in  Front  and  slightly 
from  Above. — (His). 


THE    FORMATION    OF    THE    HEART 


233 


permanent  right  and  left  atria  begins  with  the  formation  of  a  falci- 
form ridge  running  dorso-ventrally  across  the  roof  of  the  cavity. 
This  is  the  atrial  septum  or  septum  primum  (Fig.  140,  ss),  and  it 
rapidly  increases  in  size  and  thickens  upon  its  free  margin,  which 
reaches  almost  to  the  upper  border  of  the  short  atrial  canal  (Fig.  142). 
The  continuity  of  the  two  atria  is  thus  almost  dissolved,  but  is  soon 
re-established  by  the  formation  in  the  dorsal  part  of  the  septum  of 
an  opening  which  soon  reaches  a  considerable  size  and  is  known  as 


Fig.  140. — Inner  Surface  of  the  Heart  of  an  Embryo  of  10  mm. 

al,  Atrio-ventricular  thickening;  sp,  septum  spurium;  ss,  septum  primum;  sv,  septum 

ventriculi;  ve,  Eustachian  valve. — (His.) 

the  foramen  ovale  (Fig.  141,  fo).  Close  to  the  atrial  septum,  and 
parallel  with  it,  a  second  ridge  appears  in  the  roof  and  ventral  wall 
of  the  right  atrium.  This  septum  secundum  (Fig.  141,  S2)  is  of 
relatively  slight  development  in  the  human  embryo,  and  its  free 
edge,  arching  around  the  ventral  edge  and  floor  of  the  foramen 
ovale,  becomes  continuous  with  the  left  lip  of  the  fold  which  guards 
the  opening  of  the  sinus  venosus  and  with  this  forms  the  annulus 
of  Vieussens  of  the  adult  heart. 


234 


THE    FORMATION    OF    THE   HEART 


Si  Sz 


When  the  absorption  of  the  sinus  venosus  into  the  wall  of  the 
right  atrium  has  proceeded  so  far  that  the  veins  communicate 
directly  with  the  atrium,  the  vena  cava  superior  opens  into  it  at  the 
upper  part  of  the  dorsal  wall,  the  vena  cava  inferior  more  laterally, 
and  below  this  is  the  smaller  opening  of  the  coronary  sinus.     The 

upper  portion  of  the  right  lip  of  the  fold 
which  originally  surrounded  the  opening 
of  the  sinus  venosus,  together  with  the 
septum  spurium,  gradually  disappears; 
the  lower  portion  persists,  however,  and 
forms  (i)  the  Eustachian  valve  (Fig.  141, 
Ve),  guarding  the  opening  of  the  inferior 
cava  and  directing  the  blood  entering  by 
it  toward  the  foramen  ovale,  and  (2)  the 
Thebesian  valve,  which  guards  the  open- 
ing of  the  coronary  sinus.  At  first  no 
Fig.  141.— Heart  of  Embryo    veins  communicate  with  the  left  atrium, 

OF  I0.2  CM.    FROM   WHICH    HALF      ,  111  c     t        i  1 

of  the   Right  Auricle  has    but  on  the  development  of  the  lungs  and 
been  Removed.  the  establishment   of    their  vessels,   the 

fo,   Foramen   ovale;   pa,   pul-  ,  .  .,, 

monary  artery;  Su  septum  pri-    pulmonary  veins  make  connection  with 

mum;     S2,_   septum    secundum;     jt       TwQ  yejns  arise  from  each  lung   an(J 
ba,  systemic  aorta;  V,  right  ven-  ° 

tricle;  vd  and  vcs,  inferior  and    as  they  pass  toward  the  heart  they  unite 

superior  venae  cavae;  Ve,  Eusta-  ,-,  i  r     „    j  • 

chfan  valve.     .  in  pairs,  the  two  vessels  so  formed  again 

uniting  to  form  a  single  short  trunk  which 
opens  into  the  upper  part  of  the  atrium  (Fig.  142,  Vep).  As  is  the 
case  with  the  right  atrium  and  the  sinus  venosus,  the  expansion  of 
the  left  atrium  brings  about  the  absorption  of  the  short  single  trunk 
into  its  walls,  and,  the  expansion  continuing,  the  two  vessels  are  also 
absorbed,  so  that  eventually  the  four  primary  veins  open  independ- 
ently into  the  atrium. 

While  the  atrial  septa  have  been  developing  there  has  appeared 
on  the  dorsal  wall  of  the  atrial  canal  a  tubercle-like  thickening  of 
the  endocardium,  and  a  similar  thickening  also  forms  on  the  ventral 
wall.  These  endocardial  cushions  increase  in  size  and  finally  unite 
together  by  their  tips,  forming  a  complete  partition,  dividing  the 


THE    FORMATION    OF    THE   HEART 


235 


atrial  canal  into  a  right  and  left  half  (Fig.  142).  With  the  upper 
edge  of  this  partition  the  thickened  lower  edge  of  the  atrial  septum 
unites,  so  that  the  separation  of  the  atria  would  be  complete  were  it 
not  for  the  foramen  ovale. 


SM 


En.s 


Fig.   142. — Section  through  a   Reconstruction  of  the  Heart  of  a  Rabbit 

Embryo  of  10.  i  mm. 
Ad  and  Adu  Right  and  As,  left  atrium;  Bwx  and  Bw2,  lower  ends  of  the  ridges 
which  divide  the  aortic  bulb;  En,  endocardial  cushion;  En.r  and  En.s,  thickenings 
of  the  cushion;  la,  interatrial  and  Iv,  interventricular  communication;  Sv  septum 
primum;  Sd,  right  and  Ss,  left  horn  of  the  sinus  venosus;  S.iv,  ventricular  septum; 
SM,  opening  of  the  sinus  venosus  into  the  atrium;  Vd,  right  and  Vs,  left  ventricle; 
Vej,  jugular  vein;  Vep,  pulmonary  vein;  Vvd  and  Vvs,  right  and  left  limbs  of  the 
valve  guarding  the  opening  of  the  sinus  venosus. — (Born.) 

While  these  changes  have  been  taking  place  in  the  atrial  portion 
of  the  heart,  the  separation  of  the  right  and  left  ventricles  has  also 
been  progressing,  and  in  this  two  distinct  septa  take  part.  From 
the  floor  of  the  ventricular  cavity  along  the  line  of  junction  of  the 


236  THE  FORMATION  OF  THE  HEART 

right  and  left  portions  a  ridge,  composed  largely  of  muscular  tissue, 
arises  (Figs.  140  and  142),  and,  growing  more  rapidly  in  its  dorsal 
than  its  ventral  portion,  it  comes  into  contact  and  fuses  with  the 
dorsal  part  of  the  partition  of  the  atrial  canal.  Ventrally,  however, 
the  ridge,  known  as  the  ventricular  septum,  fails  to  reach  the  ventral 
part  of  the  partition ,  so  that  an  oval  foramen,  situated  just  below  the 
point  where  the  aortic  bulb  arises,  still  remains  between  the  two 
ventricles.  This  opening  is  finally  closed  by  what  it  termed  the 
aortic  septum.  This  makes  its  appearance  in  the  aortic  bulb  just  at 
the  point  where  the  first  lateral  branches  which  give  origin  to  the 
pulmonary  arteries  (see  p.  243)  arise,  and  is  formed  by  the  fusion 
of  the  free  edges  of  two  endocardial  ridges  which  develop  on  opposite 
sides  of  the  bulb.  From  its  point  of  origin  it  gradually  extends 
down  the  bulb  until  it  reaches  the  ventricle,  where  it  fuses  with 
the  free  edge  of  the  ventricular  septum  and  so  completes  the  separa- 
tion of  the  two  ventricles  (Fig.  143).  The  bulb  now  consists  of  two 
vessels  lying  side  by  side,  and  owing  to  the  position  of  the  partition 
at  its  anterior  end,  one  of  these  vessels,  that  which  opens  into  the 
right  ventricle,  is  continuous  with  the  pulmonary  arteries,  while  the 
other,  which  opens  into  the  left  ventricle,  is  continuous  with  the  rest 
of  the  vessels  which  arise  from  the  forward  continuation  of  the  bulb. 
As  soon  as  the  development  of  the  partition  is  completed,  two  grooves, 
corresponding  in  position  to  the  lines  of  attachment  of  the  partition 
on  the  inside  of  the  bulb,  make  their  appearance  on  the  outside  and 
gradually  deepen  until  they  finally  meet  and  divide  the  bulb  into  two 
separate  vessels,  one  of  which  is  the  pulmonary  aorta  and  the  other 
the  systemic  aorta. 

In  the  early  stages  of  the  heart's  development  the  muscle  bundles 
which  compose  the  wall  of  the  ventricle  are  very  loosely  arranged, 
so  that  the  ventricle  is  a  somewhat  spongy  mass  of  muscular  tissue 
with  a  relatively  small  cavity.  As  development  proceeds  the  bundles 
nearest  the  outer  surface  come  closer  together  and  form  a  compact 
layer,  those  on  the  inner  surface,  however,  retaining  their  loose 
arrangement  for  a  longer  time  (Fig.  142).  The  lower  edge  of  the 
atrial  canal  becomes  prolonged  on  the  left  side  into  one,  and  on  the 


THE    FORMATION    OF    THE   HEART 


237 


right  side  into  two,  flaps  which  project  downward  into  the  ventricular 
cavity,  and  an  additional  flap  arises  on  each  side  from  the  lower 


S.ur 


Eav.d 


Sivr 


Fig.  143. — Diagrams  of  Sections  through  the  Heart  of  Embryo  Rabbits 
to  Show  the  Mode  of  Division  of  the  Ventricles  and  of  the  Atrio-ventricular 
Orifice. 

Ao,  Aorta;  Ar.  p,  pulmonary  artery;  B,  aortic  bulb;  Bw2  and  *,  one  of  the  ridges 
which  divide  the  bulb;  Eo,  and  Eu,  upper  and  lower  thickenings  of  the  margins  of 
the  atrio-ventricular  orifice;  F.av.c,  the  original  atrio-ventricular  orifice;  F.av.d  and 
F.av.s,  right  and  left  atrio-ventricular  orifices;  Oi,  interventricular  communication; 
S.iv,  ventricular  septum;  Vd  and  Vs,  right  and  left  ventricles. — {Born.) 

edge  of  the  partition  of  the  atrial  canal,  so  that  three  flaps  occur  in 
the  right  atrio-ventricular  opening  and  two  in  the  left.     To  the 


238 


THE    FORMATION   OF    THE    HEART 


under  surfaces  of  these  flaps  the  loosely  arranged  muscular  tra- 
becular of  the  ventricle  are  attached,  and  muscular  tissue  also  occurs 
in  the  flaps.  This  condition  is  transitory,  however;  the  muscular 
tissue  of  the  flaps  degenerates  to  form  a  dense  layer  of  connective 
tissue,  and  at  the  same  time  the  muscular  trabecular  undergo  a 
condensation.  Some  of  them  separate  from  the  flaps,  which  repre- 
sent the  atrio-ventricular  valves,  and  form  muscle  bundles  which 
may  fuse  throughout  their  entire  length  with  the  more  compact 
portions  of  the  ventricular  walls,  or  else  may  be  attached  only  by 
their  ends,  forming  loops;  these  two  varieties  of  muscle  bundles 
constitute  the  trabecule  carnece  of  the  adult  heart.     Other  bundles 


Fig.  144. — Diagrams  showing  the  Development  of  the  Atjriculo-ventricular 

Valves. 

b,  Muscular  trabecule;  cht,  chordae  tendinae;  mk  and  vtk1,  valve;  pm,  musculus  papillaris; 

tc,  trabeculse  carneae;  v,  ventricle. — (From  Hertwig,  after  Gegenbaur.) 


may  retain  a  transverse  direction,  passing  across  the  ventricular 
cavity  and  forming  the  so-called  moderator  bands;  while  others,  again, 
retaining  their  attachment  to  the  valves,  condense  only  at  their  lower 
ends  to  form  the  musculi  papillares,  their  upper  portions  under- 
going conversion  into  strong  though  slender  fibrous  cords,  the 
chorda  tendinece  (Fig.  144). 

The  endocardial  lining  of  the  ventricles  is  at  first  a  simple  sac 
separated  by  a  distinct  interval  from  the  myocardium,  but  when  the 
condensation  of  the  muscle  trabecular  occurs  the  endocardium  applies 
itself  closely  to  the  irregular  surface  so  formed,  dipping  into  all  the 
crevices  between  the  trabeculse  carneae  and  wrapping  itself  around 


THE    FORMATION    OF    THE   HEART  239 

the  musculi  papillares  and  chordae  tendineae  so  as  to  form  a  complete 

lining  of  the  inner  surface  of  the  myocardium. 

The  aortic  and  pulmonary  semilunar  valves  make  their  appearance, 

before  the  aortic  bulb  undergoes  its  longitudinal  splitting,  as  four 

tubercle-like  thickenings  of  connective  tissue  situated  on  the  inner 

wall  of  the  bulb  just  where  it  arises  from  the  ventricle.     When  the 

division  of  the  bulb  occurs,  two  of  the  thickenings,  situated  on 

opposite  sides,  are  divided,  so  that  both  the 

pulmonary  and  systemic  aorta?  receive  three 

thickenings  (Fig.  145).     Later  the  thickenings 

become  hollowed  out  on  the  surfaces  directed 

away  from  the  ventricles  and  are  so  converted 

into  the  pouch-like  valves  of  the  adult. 

Changes   in   the   Heart   after  Birth. — The     T  FlG-  145-— Diagrams 

/  .  Illustrating  the  For- 

heart  when  first  formed  lies  far  forward  in  the     mation  of  the  Semi- 

neck  region  of  the  embryo,  between  the  head  £toarValves.-(G^«»- 
and  the  anterior  surface  of  the  yolk-sac,  and 
from  this  position  it  gradually  recedes  until  it  reaches  its  final 
position  in  the  thorax.  And  not  only  does  it  thus  change  its  rela- 
tive position,  but  the  direction  of  its  axes  also  changes.  For  at  an 
early  stage  the  ventricles  lie  directly  in  front  of  (i.  e.,  ventrad  to) 
the  atria  and  not  below  them  as  in  the  adult  heart,  and  this  prim- 
itive condition  is  retained  until  the  diaphragm  has  reached  its  final 
position  (see  p.  322). 

In  addition  to  these  changes  in  position,  which  are  antenatal, 
important  changes  also  occur  in  the  atrial  septum  after  birth. 
Throughout  the  entire  period  of  fetal  life  the  foramen  ovale  persists, 
permitting  the  blood  returning  from  the  placenta  and  entering  the 
right  atrium  to  pass  directly  across  to  the  left  atrium,  thence  to  the 
left  ventricle,  and  so  out  to  the  body  through  the  systemic  aorta 
(see  p.  267).  At  birth  the  lungs  begin  to  function  and  the  placental 
circulation  is  cut  off,  so  that  the  right  atrium  receives  only  venous 
blood  and  the  left  only  arterial;  a  persistence  of  the  foramen  ovale 
beyond  this  period  would  be  injurious,  since  it  would  permit  of  a 
mixture  of  the  arterial  and  venous  bloods,  and,  consequently,  it 


240  DEVELOPMENT   OF   THE  ARTERIAL   SYSTEM 

closes  completely  soon  after  birth.  The  closure  is  made  possible 
by  the  fact  that  during  the  growth  of  the  heart  in  size  the  portion  of 
the  atrial  septum  which  is  between  the  edge  of  the  foramen  ovale 
and  the  dorsal  wall  of  the  atrium  increases  in  width,  so  that  the  fora- 
men is  carried  further  and  further  away  from  the  dorsal  wall  of  the 
atrium  and  comes  to  be  almost  completely  overlapped  by  the  annulus 
of  Vieussens  (Fig.  141).  This  process  continuing,  the  dorsal  portion 
of  the  atrial  septum  finally  overlaps  the  free  edge  of  the  annulus, 
and  after  birth  the  fusion  of  the.overlapping  surfaces  takes  place  and 
the  foramen  is  completely  closed. 

In  a  large  percentage  (25  to  30  per  cent.)  of  individuals  the  fusion  of 
the  surfaces  of  the  septum  and  annulus  is  not  complete,  so  that  a  slit-like 
opening  persists  between  the  two  atria.  This,  however,  does  not  allow  of 
any  mingling  of  the  blood  in  the  two  cavities,  since  when  the  atria  contract 
the  pressure  of  the  blood  on  both  sides  will  force  the  overlapping  folds 
together  and  so  practically  close  the  opening.  Occasionally  the  growth 
of  the  dorsal  portion  of  the  septum  is  imperfect  or  is  inhibited,  in  which 
case  closure  of  the  foramen  ovale  is  impossible. 

The  Development  of  the  Arterial  System.- — It  has  been  seen 
(p.  221)  that  the  formation  of  the  blood-vessels  begins  in  the  extra- 
embryonic splanchnic  mesoderm  surrounding  the  yolk-sac  and  ex- 
tends thence  toward  the  embryo.  Furthermore,  it  has  been  seen 
that  the  vessels  appear  as  capillary  networks  from  which  definite 
stems  are  later  elaborated.  This  seems  also  to  be  the  method  of 
formation  of  the  vessels  developed  within  the  body  of  the  embryo, 
the  arterial  and  venous  stems  being  first  represented  by  a  number 
of  anastomosing  capillaries,  from  which,  by  the  enlargement  of  some 
and  the  disappearance  of  the  others,  the  definite  stems  are  formed. 

The  earliest  known  embryo  that  shows  a  blood  circulation  is 
that  described  by  Eternod  (Fig.  43).  From  the  plexus  of  vessels 
on  the  yolk-sack  two  veins  arise  which  unite  with  two  other  veins 
returniDg  from  the  chorion  by  the  belly-stalk  and  passing  forward  to 
the  heart  as  the  two  umbilical  veins  (Fig.  146,  Vu).  There  is  as  yet 
no  vitelline  vein,  the  chorionic  circulation  in  the  human  embryo 
apparently  taking  precedence  over  the  vitelline.  From  the  heart 
a  short  arterial  stem  arises,  which  soon  divides  so  as  to  form  three 


DEVELOPMENT    OF    THE   ARTERIAL    SYSTEM 


241 


branches*  passing  dorsally  on  either  side  of  the  pharynx.  The 
branches  of  each  side  then  unite  to  form  a  paired  dorsal  aorta  (dAr, 
dAs)  which  extends  caudally  and  is  continued  into  the  belly-stalk 
and  so  to  the  chorion  as  the  umbilical  arteries  (Au).  There  is  as 
yet  no  sign  of  vitelline  arteries  passing  to  the  yolk-sack,  again 
an  indication  of  the  subservience  of  the  vitelline  to  the  chorionic 
circulation  in  the  human  embryo. 


Fig.   146. — Diagram  showing  the  Arrangement  of  the  Blood-vessels  in  an 

Embryo  1.3  mm.  in  Length. 

Au,  Umbilical  artery;  All,  allantois;  Ch,  chorionic  villus;  dAr  and  dAs,  right  and  left 

dorsal  aortae;  Vu,  umbilical  veins;   Ys,  yolk-sack. — (From  Kollmann  after  Eternod.) 

In  later  stages  when  the  branchial  arches  have  appeared  the 
dorsally  directed  arteries  are  seen  to  lie  in  these,  forming  what  are 
termed  the  branchial  arch  vessels,  and  later  also  the  two  dorsal 


*  Evans  (Keibel-Mall,  Human  Embryology,  Vol.  11,  1912)  considers  two  of  these 
branches  to  be  probably  plexus  formations  rather  than  definite  stems,  since  there  is 
evidence  to  indicate  that  only  one  such  stem  exists  at  such  an  early  stage  of  development. 
16 


242 


DEVELOPMENT    OF    THE  ARTERIAL    SYSTEM 


aortae  fuse  as  far  forward  as  the  region  of  the  eighth  cervical  segment 
to  form  a  single  trunk  from  which  segmental  branches  arise. 

It  will  be  convenient  to  consider  first  the  history  of  the  vessels 
which  pass  dorsally  in  the  branchial  arches.     Altogether,  six  of  these 
vessels  are  developed,  the  fifth  being  rudimentary  and  transitory,  and 
when  fully  formed  they  have  an  arrangement  which  may  be  under- 
stood  from   the   diagram    (Fig. 
147).     This  arrangement  repre- 
sents a  condition  which  is  per- 
manent in  the  lower  vertebrates. 
In  the  fishes  the  respiration  is 
performed    by    means    of    gills 
developed   upon   the    branchial 
arches,  and  the  heart  is  an  organ 
which  receives  venous  blood  from 
the  body  and  pumps  it  to  the 
gills,  in  which  it  becomes  arte- 
rialized  and  is  then  collected  into 
the  dorsal  aortae,  which  distrib- 
ute it  to  the  body.     But  in  terres- 
trial animals,  with  the  loss  of  the 
gills  and  the  development  of  the 
lungs  as  respiratory  organs,  the 
capillaries  of  the  gills  disappear 
and    the    afferent    and    efferent 
branchial  vessels    become    con- 
tinuous,   the    condition    repre- 
sented in  the  diagram  resulting. 
But  this  condition  is  merely  temporary  in  the  mammalia  and 
numerous  changes  occur  in  the  arrangement  of  the  vessels  before 
the  adult  plan  is  realized.     The  first  change  is  a  disappearance  of 
the  vessel  of  the  first  arch,  the  ventral  stem  from  which  it  arose  being 
continued  forward  to  form  the  temporal  arteries,  giving  off  near  the 
point  where  the  branchial  vessel  originally  arose  a  branch  which 
represents  the  internal  maxillary  artery  in  part,  and  possibly  also  a 


Fig.  147. — Diagram  Illustrating  the 
Primary  Arrangement  of  the  Bran- 
chial Arch  Vessels. 

a,  aorta;  db,  aortic  bulb;  ec,  external 
carotid;  ic,  internal  carotid;  sc,  subclavian; 
I-VI,  branchial  arch  vessels. 


'     DEVELOPMENT    OF    THE   ARTERIAL    SYSTEM  243 

second  branch  which  represents  the  external  maxillary  (His). 
A  little  later  the  second  branchial  vessel  also  degenerates  (Fig.  148), 
a  branch  arising  from  the  ventral  trunk  near  its  former  origin, 
possibly  representing  the  future  lingual  artery  (His),  and  then  the 
portion  of  the  dorsal  trunk  which  intervenes  between  the  third  and 
fourth  branchial  vessels  vanishes,  so  that  the  dorsal  trunk  anterior 
to  the  third  branchial  arch  is  cut  off  from  its  connection  with  the 
dorsal  aorta  and  forms,  together  with  the  vessel  of  the  third  arch,  the 
internal  carotid,  while  the  ventral  trunk,  anterior  to  the  point  of 


Fig.  148. — Arteriat,  System  of  an  Embryo  of  10  mm. 

Ic,  Internal  carotid;  P,  pulmonary  artery;  Ve,  vertebral  artery;  III  to  VI,  persistent 

branchial  vessels. — (His.) 

origin  of  the  third  vessel,  becomes  the  external  carotid,  and  the  por- 
tion which  intervenes  between  the  third  and  fourth  vessels  becomes 
the  common  carotid  (Fig.  149). 

The  rudimentary  fifth  vessel,  like  the  first  and  second,  disappears, 
but  the  fourth  persists  to  form  the  aortic  arch,  there  being  at  this 
stage  of  development  two  complete  aortic  arches.  From  the 
sixth  vessel  a  branch  arises  which  passes  backward  to  the  lungs, 
forming  the  pulmonary  artery,  and  the  portion  of  the  vessel  of  the 
right  side  which  intervenes  between  this  and  the  aortic  arch  dis- 
appears, while  the  corresponding  portion  of  the  left  side  persists 


244 


DEVELOPMENT    OF    THE   ARTERIAL   SYSTEM 


until  after  birth,  forming  the  ductus  arteriosus  {ductus  Botalli)  (Fig. 
149).  When  the  longitudinal  division  of  the  aortic  bulb  occurs 
(p.  236),  the  septum  is  so  arranged  as  to  place  the  sixth  arch  in 
communication  with  the  right  ventricle  and  the  remaining  vessels 
in  connection  with  the  left  ventricle,  the  only  direct  communication 

between    the     systemic    and 
ec  pulmonary  vessels  being   by 

way  of  the  ductus  arteriosus, 
whose  significance  will  be  ex- 
plained later  (p.  267). 

One  other  change  is  still 
necessary  before  the  vessels 
acquire  the  arrangement 
which  they  possess  during 
fetal  life,  and  this  consists  in 
the  disappearance  of  the 
lower  portion  of  the  right 
aortic  arch  (Fig.  149),  so  that 
the  left  arch  alone  forms  the 
connection  between  the  heart 
and  the  dorsal  aorta.  The 
upper  part  of  the  right  aortic 
arch  persists  to  form  the  prox- 
imal part  of  the  right  sub- 
clavian artery,  the  portion  of 
the  ventral  trunk  which  unites 
the  arch  with  the  aortic  bulb 
becoming  the  innominate 
artery. 

From  the  entire  length  of  the  thoracic  aorta,  and  in  the  embryo 
from  the  aortic  arches,  lateral  branches  arise  corresponding  to  each 
segment  and  accompanying  the  segmental  nerves.  The  first  of 
these  branches  arises  just  below  the  point  of  union  of  the  vessel 
of  the  sixth  arch  with  the  dorsal  trunk  and  accompanies  the  hypo- 
glossal nerve  (Fig.  150,  h),  and  that  which  accompanies  the  seventh 


Fig.  149. — Diagram  Illustrating  the 
changes  in  the  branchial  arch  vessels. 

a,  Aorta;  da,  ductus  arteriosus;  ec,  external 
carotid;  ic,  internal  carotid;  pa,  pulmonary  ar- 
tery; sc,  subclavian;  I- VI,  aortic  arch  vessels. 


DEVELOPMENT  OF  THE  ARTERIAL  SYSTEM 


245 


cervical  nerve  arises  just  above  the  point  of  union  of  the  two  aortic 
arches  (Fig.  150,  s),  and  extends  out  into  the  limb  bud,  forming  the 
subclavian  artery.* 

Further  down  twelve  pairs  of  lateral  branches,  arising  from  the 
thoracic  portion  of  the  aorta,  rep- 
resent the  intercostal  arteries, 
and  still  lower  four  pairs  of  lum- 
bar arteries  are  formed,  the  fifth 
lumbars  being  represented  by 
two  large  branches,  the  common 
iliacs,  which  seem  from  their  size 
to  be  the  continuations  of  the 
aorta  rather  than  branches  of  it. 
The  true  continuation  of  the 
aorta  is,  however,  the  middle  sa- 
cral artery,  which  represents  in 
a  degenerated  form  the  caudal 
prolongation  of  the  aorta  of 
other  mammals,  and,  like  this, 
gives  off  lateral  branches  corre- 
sponding to  the  sacral  segments. 

In  addition  to  the  segmental      FlG.  I50.— diagram  showing  the  Re- 

lateral    branches    arising    from    nations  op  the  Lateral  Branches  to 

0  the  Aortic  Arches. 

the    aorta,     Visceral     branches,         EC>  External  carotid;  h,   lateral  branch 

Which  have    their    origin    rather  cacompanying  the  hypoglossal  nerve;   IC, 

°  internal  carotid;  ICo,  intercostal;  IM,  m- 

from    the    Ventral    surface,    also  ternal  mammary;  s,  subclavian;  v,  verte- 

^„„,,~       TV,     ~™u„mr,    ~t    -    mm  bral;  I  to  VIII,  lateral  cervical  branches; 

OCCUr.      In    embryos    of    5    mm.  I;  2)  lateral  thoracic  branches. 

these  branches  are  arranged  in 

a  segmental  manner  in  threes,  a  median  unpaired  vessel  passing 
to  the  digestive  tract  and  a  pair  of  more  lateral  branches 
passing  to  the  mesonephros  (see  p.  339)  corresponding  to  each  of 
the   paired   branches  passing  to  the  body  wall    (Fig.    151).     As 

*  It  must  be  remembered  that  the  right  subclavian  of  the  adult  is  more  than  equiva- 
lent to  the  left,  since  it  represents  the  fourth  branchial  vessel  +  a  portion  of  the  dorsal 
longitudinal  trunk  +  the  lateral  segmental  branch  (see  Fig.  142). 


246 


DEVELOPMENT   OF    THE   ARTERIAL    SYSTEM 


development  proceeds  the  great  majority  of  these  visceral 
branches  disappear,  certain  of  the  lateral  ones  persisting,  however, 
to  form  the  renal,  internal  spermatic,  and  hypogastric  arteries  of 
the  adult,  while  the  unpaired  branches  are  represented  only  by  the 
c celiac  artery  and  the  superior  and  inferior  mesenteries.  The 
superior  mesenteric  artery  is  the  adult  representative  of  the  vitelline 
artery  of  the  embryo  and  arises  from  the  aorta  by  two,  three  or  more 
roots,  which  correspond  to  the  fifth,  fourth  and  higher  thoracic 


Fig.  151. — Diagram  showing  the  Arrangement  of  the  Segmental  Branches 

arising  from  the  aorta. 
A,  Aorta;  B,  lateral  somatic  branch;  c,  lateral  visceral  branch;  D,  median  visceral 

branch;  E,  peritoneum. 

segments.  Later,  all  but  the  lowest  of  the  roots  disappear  and  the 
persisting  one  undergoes  a  downward  migration  in  accordance  with 
the  recession  of  the  diaphragm  and  viscera  (see  p.  322),  until  in 
embryos  of  17  mm.  it  lies  opposite  the  first  lumbar  segment.  Simi- 
larly the  cceliac  and  inferior  mesenteric  arteries,  which  when  first 
recognizable  in  embryos  of  9  mm.  correspond  with  the  fourth  and 
twelfth  thoracic  segments  respectively,  also  undergo  a  secondary 
downward  migration,  the  cceliac  artery  in  embryos  of  17  mm.  arising 


DEVELOPMENT    OF    THE   ARTERIAL    SYSTEM 


247 


opposite  the  twelfth  thoracic  and  the  inferior  mesenteric  opposite 
the  third  lumbar  segment. 

The  umbilical  arteries  of  the  embryo  seem  at  first  to  be  the  direct 
continuations  of  the  dorsal  aortas  (Fig.  146),  but  as  development 
proceeds  they  come  to  arise  from  the  aorta  opposite  the  third 
lumbar  segment,  where  they  are  in  line  with  the  lateral  visceral 
segmental  branches.  They  pass  ventral  to  the  Wolffian  duct  (see 
p.  339)  and  are  continued  out 
along  with  the  allantois  to  the 
chorionic  villi.  Later  this 
original  stem  is  joined,  not  far 
from  its  origin,  by  what  ap- 
pears to  be  the  lateral  somatic 
branch  of  the  fifth  lumbar  seg- 
ment, whereupon  the  proximal 
part  of  the  original  umbilical 
vessel  degenerates  and  the  um- 
bilical comes  to  arise  from  the 
somatic  branch,  which  is  the 
common  iliac  artery  of  adult 
anatomy  (Fig.  152).  Hence 
it  is  that  this  vessel  in  the  adult 
gives  origin  both  to  branches 
such  as  the  external  iliac,  the 
gluteal,  the  sciatic  and  the  in- 
ternal pudendal,  which  are 
distributed  to  the  body  walls 

or  their  derivatives,  and  to  others,  such  as  the  vesical,  inferior  haemor- 
rhoidal  and  uterine,  which  are  distributed  to  the  pelvic  viscera.  At 
birth  the  portions  of  the  umbilical  arteries  beyond  the  umbilicus  are 
severed  when  the  umbilical  cord  is  cut,  and  their  intra-embryonic 
portions,  which  have  been  called  the  hypogastric  arteries,  quickly 
undergo  a  reduction  in  size.  Their  proximal  portions  remain 
functional  as  the  superior  vesical  arteries,  carrying  blood  to  the 
urinary  bladder,   but  the  portions  which  intervene   between   the 


Fig.  152. — Diagram  Illustrating  the 
Development  of  the  Umbilical  Arteries. 

A,  Aorta;  CIl,  common  iliac;  Ell,  exter- 
nal iliac;  G,  gluteal;  III,  internal  iliac;  IP, 
internal  pudic;  IV,  inferior  vesical;  Sc,  scia- 
tic; U,  umbilical;  U',  primary  proximal  por- 
tion of  the  umbilical;  wd,  Wolffian  duct. 


248  DEVELOPMENT    OF    THE  ARTERIAL    SYSTEM 

bladder  and  the  umbilicus  become  reduced  to  solid  cords,  forming 
the  obliterated  hypogastric  arteries  of  adult  anatomy. 
f~  In  its  general  plan,  accordingly,  the  arterial  system  may  be 
regarded  as  consisting  of  a  pair  of  longitudinal  vessels  which  fuse 
together  throughout  the  greater  portion  of  their  length  to  form 
the  dorsal  aorta,  from  which  there  arise  segmentary  arranged 
lateral  somatic  branches  and  ventral  and  lateral  visceral  branches. 
With  the  exception  of  the  aortic  trunks  (together  with  their  anterior 
continuations,  the  internal  carotids)  and  the  external  carotids,  no 
longitudinal  arteries  exist  primarily.  In  the  adult,  however,  several 
longitudinal  vessels,  such  as  the  vertebrals,  internal  mammary, 
and  epigastric  arteries,  exist.  The  formation  of  these  secondary 
longitudinal  trunks  is  the  result  of  a  development  between  adjacent 
vessels  of  anastomoses,  which  become  larger  and  more  important 
blood-channels  than  the  original  vessels. 

At  an  early  stage  each  of  the  lateral  branches  of  the  dorsal  aorta 
gives  off  a  twig  which  passes  forward  to  anastomose  with  a  back- 
wardly  directed  twig  from  the  next  anterior  lateral  branch,  so  as  to 
form  a  longitudinal  chain  of  anastomoses  along  each  side  of  the 
neck.  In  the  earliest  stage  at  present  known  the  chain  starts  from 
the  lateral  branch  corresponding  to  the  first  cervical  (suboccipital) 
segment  and  extends  forward  into  the  skull  through  the  foramen 
magnum,  terminating  by  anastomosing  with  the  internal  carotid. 
To  this  original  chain  other  links  are  added  from  each  of  the 
succeeding  cervical  lateral  branches  as  far  back  as  the  seventh 
(Figs.  150  and  153).  But  in  the  meantime  the  recession  of  the 
heart  toward  the  thorax  has  begun,  with  the  result  that  the  common 
carotid  stems  are  elongated  and  the  aortic  arches  are  apparently 
shortened  so  that  the  subclavian  arises  on  the  left  side  almost 
opposite  the  point  where  the  aorta  was  joined  by  the  sixth  branchial 
vessel.  As  this  apparent  shortening  proceeds,  the  various  lateral 
branches  which  give  rise  to  the  chain  of  anastomoses,  with  the 
exception  of  the  seventh,  disappear  in  their  proximal  portions  and 
the  chain  becomes  an  independent  stem,  the  vertebral  artery,  arising 
from  the  seventh  lateral  branch,  which  is  the  subclavian. 


DEVELOPMENT    OF    THE   ARTERIAL   SYSTEM 


249 


The  recession  of  the  heart  is  continued  until  it  lies  below  the 
level  of  the  upper  intercostal  arteries,  and  the  upper  two  of  these, 
together  with  the  last  cervical  branch  on  each  side,  lose  their  connec- 
tion with  the  dorsal  aorta,  and,  sending  off  anteriorly  and  posteriorly 


-A.VCK 


Fig.  153. — The  Development  of  the  Vertebral  Artery  in  a  Rabbit  Embryo 

of  Twelve  Days. 

IIIA.B  to  VIA.B,  Branchial  arch  vessels;  Ap,  pulmonary  artery.  A.v.c.b  and 
A.v.cv,  cephalic  and  cervical  portions  of  the  vertebral  artery;  A.s,  subclavian;  C.d 
and  C.v  internal  and  external  carotid ;  ISp.G,  spinal  ganglion. — (Hochstetter.) 


anastomosing  twigs,  develop  a  short  longitudinal  stem,  the  superior 
intercostal,  which  opens  into  the  subclavian. 

The    intercostals    and    their    abdominal    representatives,     the 


250 


DEVELOPMENT    OF  ARTERIES    OF    LIMBS 


lumbars  and  iliacs,  also  give  rise  to  longitudinal  anastomosing 
twigs  near  their  ventral  ends  (Fig.  154),  and  these  increasing  in 
size  give  rise  to  the  internal  mammary  and  inferior  epigastric  arteries, 
which  together  form  continuous  stems  extending  from  the  sub- 
clavian to  the  external  iliacs  in  the  ventral  abdominal  walls.  The 
superficial  epigastrics  and  other  secondary  longitudinal  vessels  are 
formed  in  a  similar  manner. 

The  Development  of  the  Arteries  of  the  Limbs. — The  earliest 
stages  in  the  development  of  the  limb  arteries  are  unknown  in  man, 


Fig.  154, 


-Embryo  of  13  mm.  showing  the  Mode  of  Development  of  the  Internal 
Mammary  and  Deep  Epigastric  Arteries. — (Mall.) 


but  it  has  been  found  that  in  the  mouse  the  primary  supply  of  the 
anterior  limb  bud  is  from  five  branches  arising  from  the  sides  of  the 
aorta.  These  anastomose  to  form  a  plexus  from  which  later  a  single 
stem,  the  subclavian  artery,  is  elaborated,  occupying  the  position 
of  the  seventh  cervical  segmental  vessel,  the  remaining  branches  of 
the  plexus  having  disappeared.     The  common  iliac  artery  similarly 


DEVELOPMENT    OF  ARTERIES    OF   LIMBS  25 1 

represents  the  fifth  lumbar  segmental  artery,  but  whether  or  not  it 
also  is  elaborated  from  a  plexus  is  as  yet  unknown. 

The  later  history  of  the  limb  arteries  is  also  but  imperfectly 
known  and  one  must  rely  largely  upon  the  facts  of  comparative 
anatomy  and  on  the  anomalies  that  occur  in  the  adult  for  indications 
of  what  the  development  is  likely  to  be.  The  comparative  evidence 
indicates  the  existence  of  several  stages  in  the  development  of  the 
limb  vessels,  and  so  far  as  embryological  observations  go  they 
confirm  the  conclusions  drawn  from  this  source,  although  the  various 
stages  show  apparently  a  great  amount  of  overlapping  owing  to  a 
concentration  of  the  developmental  stages.  In  the  simplest  arrange- 
ment the  subclavian  is  continued  as  a  single  trunk  along  the  axis 
of  the  limb  as  far  as  the  carpus,  where  it  divides  into  digital  branches 
for  the  fingers.  In  its  course  through  the  forearm  it  lies  in  the 
interval  between  the  radius  and  ulna,  resting  on  the  interosseous 
membrane,  and  in  this  part  of  its  course  it  may  be  termed  the  arteria 
interossea.  In  the  second  stage  a  new  artery  accompanying  the 
median  nerve  appears,  arising  from  the  main  stem  or  brachial 
artery  a  little  below  the  elbow-joint.  This  may  be  termed  the 
arteria  mediana,  and  as  it  develops  the  arteria  interossea  gradually 
diminishes  in  size,  becoming  finally  the  small  volar  interosseous 
artery  of  the  adult  (Fig.  155),  and  the  median,  uniting  with  its 
lower  end,  takes  from  it  the  digital  branches  and  becomes  the  prin- 
cipal stem  of  the  forearm. 

A  third  stage  is  then  ushered  in  by  the  appearance  of  a  branch 
from  the  brachial  which  forms  the  arteria  ulnaris,  and  this,  passing 
down  the  ulnar  side  of  the  forearm,  unites  at  the  wrist  with  the 
median  to  form  a  superficial  palmar  arch  from  which  the  digital 
branches  arise.  A  fourth  stage  is  marked  by  the  diminution  of  the 
median  artery  until  it  finally  appears  to  be  ,a  small  branch  of  the 
interosseous,  and  at  the  same  time  there  develops  from  the  brachial, 
at  about  the  middle  of  the  upper  arm,  what  is  known  as  the  arteria 
radialis  superficial  (Fig.  155,  rs).  This  extends  down  the  radial 
side  of  the  forearm,  following  the  course  of  the  radial  nerve,  and  at 
the  wrist  passes  upon  the  dorsal  surface  of  the  hand  to  form  the 


252 


DEVELOPMENT   OF  ARTERIES    OP   LIMBS 


dorsal  digital  arteries  of  the  thumb  and  index  finger.  At  first  this 
artery  takes  no  part  in  the  formation  of  the  palmar  arches,  but  later 
it  gives  rise  to  the  superficial  volar  branch,  which  usually  unites 
with  the  superficial  arch,  while  from  its  dorsal  portion  a  perforating 
branch  develops  which  passes  between  the  first  and  second  meta- 


r 


Fig.  155. — Diagrams  showing  an  Early  and  a  Late  Stage  in  the  Development 

of  the  Arteries  of  the  Arm. 

b,  Brachial;  i,  interosseous;  m,  median;  r,  radial;  rs,  superficial  radial;  u,  ulnar. 


carpal  bones  and  unites  with  a  deep  branch  of  the  ulnar  to  form  the 
deep  arch.  The  fifth  or  adult  stage  is  reached  by  the  development 
from  the  brachial  below  the  elbow  of  a  branch  (Fig.  155,  r)  which 
passes  downward  and  outward  to  unite  with  the  superficial  radial, 
whereupon  the  upper  portion  of  that  artery  degenerates  until  it  is 


DEVELOPMENT   OF  ARTERIES    OF   LIMBS  253 

represented  only  by  a  branch  to  the  biceps  muscle  (Schwalbe),  while 
the  lower  portion  persists  as  the  adult  radial. 

The  various  anomalies  seen  in  the  arteries  of  the  forearm  are,  as  a 
rule,  due  to  the  more  or  less  complete  persistence  of  one  or  other  of  the 
stages  described  above,  what  is  described,  for  instance,  as  the  high  branch- 
ing of  the  brachial  being  the  persistence  of  the  superficial  radial. 

In  the  leg  there  is  a  noticeable  difference  in  the  arrangement  of 
the  arteries  from  what  occurs  in  the  arm,  in  that  the  principal  artery 
of  the  thigh,  the  femoral,  does  not  accompany  the  principal  nerve, 
the  sciatic.  This  difference  is  apparently  secondary,  but,  as  in  the 
case  of  the  upper  limb,  it  is  necessary  to  rely  largely  on  the  facts  of 
comparative  anatomy  and  on  anomalies  which  occur  in  the  human 
body  for  an  idea  of  the  probable  development  of  the  arteries  of  the 
lower  limb.  It  has  already  been  seen  that  the  common  iliac  artery 
is  to  be  regarded  as  a  lateral  branch  of  the  dorsal  aorta,  and  in  the 
simplest  condition  of  the  limb  arteries  its  continuation,  the  anterior 
division  of  the  hypogastric,  passes  down  the  leg  as  a  well-developed 
sciatic  artery  as  far  as  the  ankle  (Fig.  156,5).  At  the  knee  it  occupies 
the  position  of  the  popliteal  of  adult  anatomy,  and  below  the  knee 
gives  off  a  branch  corresponding  to  the  anterior  tibial  (at)  which, 
passing  forward  to  the  extensor  surface  of  the  leg,  quickly  loses  itself 
in  the  extensor  muscles.  The  main  artery  continues  downward  on 
the  interosseous  membrane,  and  some  distance  above  the  ankle 
divides  into  a  strong  anterior  and  a  weaker  posterior  branch;  the 
former  perforates  the  membrane  and  is  continued  down  the  extensor 
surface  of  the  leg  to  form  the  lower  part  of  the  anterior  tibial  and 
the  dorsalis  pedis  arteries,  while  the  latter,  passing  upon  the  plantar 
surface  of  the  foot,  is  lost  in  the  plantar  muscles.  At  this  stage  the 
external  iliac  is  a  secondary  branch  of  the  common  iliac,  being  but 
poorly  developed  and  not  extending  as  far  as  the  knee. 

In  the  second  stage  the  external  iliac  artery  increases  in  size  until  it 
equals  the  sciatic,  and  it  now  penetrates  the  adductor  magnus 
muscle  and  unites  with  the  popliteal  portion  of  the  sciatic.  Before 
doing  this,  however,  it  gives  off  a  strong  branch  (sa)  which  accom- 
panies the  long  saphenous  nerve  down  the  inner  side  of  the  leg,  and, 


254 


DEVELOPMENT    OF  ARTERIES    OE   LIMBS 


passing  behind  the  internal  malleolus,  extends  upon  the  plantar 
surface  of  the  foot,  where  it  gives  rise  to  the  digital  branches.  From 
this  arrangement  the  adult  condition  may  be  derived  by  the  con- 
tinued increase  in  size  of  the  external  iliac  and  its  continuation,  the 
femoral  (/),  accompanied  by  a  reduction  of  the  upper  portion  of  the 
sciatic  and  its  separation  from  its  popliteal  portion  (p)  to  form  the 
inferior  gluteal  artery  of  the  adult.     The   continuation  of  the  popli- 


n 


i,° 


p 


pe 


f 


at 


\s 


P 


pe 


t\ 


Pt 


C 


Fig.  156. — Diagrams  Illustrating  Stages  in  the  Development  of  the  Arteries 

of  the  Leg. 

at,  Anterior  tibial;  dp,  dorsalis  pedis;/,  femoral;  p,  popliteal;  pe,  peroneal  pt,  posterior 

tibial;  s,  sciatic  (inferior  gluteal);  sa,  saphenous. 

teal  down  the  leg  is  the  peroneal  artery  (pe)  and  the  upper  perforating 
branch  of  this  unites  with  the  lower  one  to  form  a  continuous  ante- 
rior tibial,  the  lower  connection  of  which  with  the  peroneal  persists 
in  part  as  the  anterior  peroneal  artery.  A  new  branch  arises  from 
the  upper  part  of  the  peroneal  and  passes  down  the  back  of  the  leg 


DEVELOPMENT    OF  THE    VENOUS    SYSTEM  255 

to  unite  with  the  lower  part  of  the  arteria  saphena,  forming  the 
posterior  tibial  artery  (pt),  and  the  upper  part  of  the  saphenous 
becomes  much  reduced,  persisting  as  the  superficial  branch  of  the 
art.  genu  suprema  and  a  rudimentary  chain  of  anastomoses  which 
accompany  the  long  saphenous  nerve. 

The  Development  of  the  Venous  System. — The  earliest  veins 
to  develop  are  those  which  accompany  the  first-formed  arteries,  the 
umbilicals,  but  it  will  be  more  convenient  to  consider  first  the  veins 
which  carry  the  blood  from  the  body  of  the  embryo  back  to  the 
heart.  These  make  their  appearance,  while  the  heart  is  still  in  the 
pharyngeal  region,  as  two  pairs  of  longitudinal  trunks,  the  anterior 
and  posterior  cardinal  veins,  into  which  lateral  branches,  arranged 
more  or  less  segmentally,  open.  The  anterior  cardinals  appear 
somewhat  earlier  than  the  posterior  and  form  the  internal  jugular 
veins  of  adult  anatomy.  Each  vein  extends  forward  from  the  heart 
at  the  side  of  the  notochord  and  is  continued  on  the  under  surface 
of  the  brain,  lying  medial  to  the  roots  of  the  cranial  nerves.  Later 
sprouts  arising  from  the  vein  form  loops  around  the  nerve  roots  and 
the  portion  of  the  loops  formed  by  the  original  vein  then  disappear, 
so  that  the  vessel  now  lies  lateral  to  the  nerve  roots,  except  in  the  case 
of  the  trigeminus,  where  the  original  vessel  persists  to  form  the 
cavernous  sinus.  From  the  vena  capitis  lateralis  so  formed  three 
veins,  an  anterior,  a  middle  and  a  posterior  cerebral,  pass  to  the 
brain,  the  anterior  cerebral  together  with  the  ophthalmic  vein  opening 
into  the  anterior  end  of  the  cavernous  sinus,  the  middle  cerebral  into 
the  posterior  extremity  of  the  same  sinus  and  the  posterior  cerebral 
into  the  vena  capitis  lateralis  behind  the  ear  vesicle  (Fig.  157).  The 
branches  of  the  anterior  cerebral  vein  extending  over  the  cerebral  hem- 
isphere unite  with  their  fellows  of  the  opposite  side  to  form  a  longitu- 
dinal trunk,  the  superior  sagittal  sinus,  lying  between  the  two  cere- 
bral hemispheres.  At  first  this  sinus  drains  by  way  of  the  anterior 
cerebral  vein  (Fig.  158,  A),  but  as  the  cerebral  hemispheres  increase 
in  size  it  is  gradually  carried  backward  and  makes  connections  first 
with  the  middle  cerebral  and  later  with  the  posterior  cerebral  vein 
(Fig.  158,  B  and  C),  each  of  these  becoming  in  turn  the  principal 


256 


DEVELOPMENT   OF   THE    VENOUS    SYSTEM 


drainage  of  the  sinus.  The  connections  which  join  the  veins  to  the 
sinus  become  the  proximal  portion  of  the  transverse  sinus,  the  poste- 
rior cerebral  vein  itself  becoming  the  distal  portion,  the  middle 
cerebral  vein  becomes  the  superior  petrosal  sinus,  while  the  anterior 
cerebral  vein  persists  as  the  middle  cerebral  vein  of  adult  anatomy 


m  vci  vcv 


Fig.  157. — Reconstruction  of  the  Head  of  a  Human  Embryo  of  9  mm.  showing 

the  Cerebral  Veins. 
acv,  Anterior  cerebral  vein;   au,  auditory  vesicle;   cs,  cavernous  sinus;  fa,  facial 
nerve;  mcv,  middle  cerebral  vein;  pcv,  posterior  cerebral  vein;  tr,  trigeminal  nerve; 
vcl,  lateral  cerebral  vein. — {Mall.) 


(Fig.  158,  C).  Additional  sprouts  from  the  terminal  portion  of  the 
superior  sagittal  sinus  give  rise  to  the  straight  and  inferior  sagittal 
sinuses,  and,  after  the  disappearacne  of  the  vena  capitis  lateralis,  a 
new  stem  develops  between  the  cavernous  and  transverse  sinuses, 
passing  medial  to  the  ear  vesicle,  and  forms  the  inferior  petrosal 
sinus  (Fig.  158,  C).     This  joins  the  transverse  sinus  at  the  jugular 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM 


257 


foramen  and  from  this  junction  onward  the  anterior  cardinal  vein 
may  now  be  termed  the  internal  jugular  vein. 

Passing  backward  from  the  jugular  foramen  the  internal  jugular 
veins  unite  with  the  posterior  cardinals  to  form  on  each  side  a  common 
trunk,  the  ductus  Cuvieri,  and  then  passing  transversely  toward  the 
median  line  open  into  the  sides  of  the  sinus  venosus.  So  long  as  the 
heart  retains  its  original  position  in  the  pharyngeal  region  the  jugular 


Fig.  158. — Diagrams  showing  the  Arrangement  of  the  Cerebral  Veins  in 
Embryos  of  (A)  the  Fifth  Week,  (B)  the  Beginning  of  the  Third  Month  and 
in  (C)  an  Older  Fetus. 

acv,  Anterior  cerebral  vein;  cs,  cavernous  sinus;  Us,  inferior  sagittal  sinus;  Inf. 
Pet.,  inferior  petrosal  sinus;  Is,  transverse  sinus;  ov,  ophthalmic  vein;  sis,  superior 
sagittal  sinus;  sps,  spheno-parietal  sinus;  sr,  straight  sinus;  55,  middle  cerebral  vein 
(Sylvian);  sup.  pet,  superior  petrosal  sinus;  th,  torcular  Herophili;  v,  trigeminal  nerve; 
vca,  anterior  cerebral  vein;  vol.  lateral  cerebral  vein;  vcm,  middle  cerebral  vein;  vcp, 
posterior  cerebral  vein;  vg,  vein  of  Galen;  vj,  internal  jugular. — (Mall.) 

is  a  short  trunk  receiving  lateral  veins  only  from  the  uppermost  seg- 
ments of  the  neck  and  from  the  occipital  segments,  the  remaining 
segmental  veins  opening  into  the  inferior  cardinals.  As  the  heart 
recedes,  however,  the  jugulars  become  more  and  more  elongated 

17 


258 


DEVELOPMENT   OF   THE    VENOUS   SYSTEM 


and  the  cervical  lateral  veins  shift  their  communication  from  the 
cardinals  to  the  jugulars,  until,  when  the  subclavians  have  thus 
shifted,  the  jugulars  become  much  larger  than  the  cardinals.  When 
the  sinus  venosus  is  absorbed  into  the  wall  of  the  right  auricle,  the 
course  of  the  left  Cuvierian  duct  becomes  a  little  longer  than  that 
of  the  right,  and  from  the  left  jugular,  at  the  point  where  it  is  joined 
by  the  left  subclavian,  a  branch  arises  which  extends  obliquely  across 
to  join  the  right  jugular,  forming  the  left  innominate  vein.  When 
this  is  established,  the  connection  between  the  left  jugular  and 
Cuvierian  duct  is  dissolved,  the  blood  from  the  left  side  of  the  head 
and  neck  and  from  the  left  subclavian  vein  passing  over  to  empty 


Fig.  159. — Diagrams  showing  the  Development  of  the  Superior  Vena  Cava. 
a,  Azygos  vein;  cs,  coronary  sinus;  ej,  external  jugular;  h,  hepatic  vein;  ij,  internal 
jugular;  inr  and  inl,  right  and  left  innominate  veins;  s,  subclavian;  vci  and  vcs,  inferior 
and  superior  venae  cava?. 

into  the  right  jugular,  whose  lower  end,  togethei  with  the  right 
Cuvierian  duct,  thus  becomes  the  superior  vena  cava.  The  left 
Cuvierian  duct  persists,  forming  with  the  left  horn  of  the  sinus 
venosus  the  coronary  sinus  (Fig.  159). 

The  external  jugular  vein  develops  somewhat  later  than  the 
internal.  The  facial  vein,  which  primarily  forms  the  principal 
affluent  of  this  stem,  passes  at  first  into  the  skull  along  with  the  fifth 
nerve  and  communicates  with  the  internal  jugular  system,  but  later 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM  259 

this  original  communication  is  broken  and  the  facial  vein,  uniting 
with  other  superficial  veins,  passes  over  the  jaw  and  extends  down 
the  neck  as  the  external  jugular.  Later  still  the  facial  anastomoses 
with  the  ophthalmic  at  the  inner  angle  of  the  eye  and  also  makes 
connections  with  the  internal  jugular  just  after  it  has  crossed  the  jaw, 
and  so  the  adult  condition  is  acquired. 

It  is  interesting  to  note  that  in  many  of  the  lower  mammals  the  external 
jugular  becomes  of  much  greater  importance  than  the  internal,  the  latter 
in  some  forms,  indeed,  eventually  disappearing  and  the  blood  from  the 
interior  of  the  skull  emptying  by  means  of  anastomoses  which  have 
developed  into  the  external  jugular  system.  In  man  the  primitive  con- 
dition is  retained,  but  indications  of  a  transference  of  the  intracranial 
blood  to  the  external  jugular  are  seen  in  the  emissary  veins. 

The  posterior  cardinal  veins,  or,  as  they  may  more  simply  be 
termed,  the  cardinals,  extend  backward  from  their  union  with  the 
jugulars  along  the  sides  of  the  vertebral  column,  receiving  veins 
from  the  mesentery  and  also  from  the  various  lateral  segmental 
veins  of  the  neck  and  trunk  regions,  with  the  exception  of  that  of 
the  first  cervical  segment  which  opens  into  the  jugular.  Later, 
however,  as  already  described  (p.  258),  the  cervical  veins  shift  to 
the  jugulars,  as  do  also  the  first  and  second  thoracic  (intercostal) 
veins,  but  the  remaining  intercostals,  together  with  the  lumbars 
and  sacrals,  continue  to  open  into  the  cardinals.  In  addition,  the 
cardinals  receive  in  early  stages  the  veins  from  the  primitive  kidneys 
(meson ephros),  which  are  exceptionally  large  in  the  human  embryo, 
but  when  they  are  replaced  later  on  by  the  permanent  kidneys 
(metanephros)  their  afferent  veins  undergo  a  reduction  in  number 
and  size,  and  this,  together  with  the  shifting  of  the  upper  lateral  veins, 
produces  a  marked  diminution  in  the  size  of  the  cardinals.  The 
changes  by  which  they  acquire  their  final  arrangement  are,  however, 
so  intimately  associated  with  the  development  of  the  inferior  vena 
cava  that  their  description  may  be  conveniently  postponed  until  the 
history  of  the  vitelline  and  umbilical  veins  has  been  presented. 

The  vitelline  veins  are  two  in  number,  a  right  and  a  left,  and  pass 
in  along  the  yolk-stalk  until  they  reach  the  embryonic  intestine, 
along  the  sides  of  which  they  pass  forward  to  unite  with  the  corre- 


20O 


DEVELOPMENT    OF   THE   VENOUS    SYSTEM 


sponding  umbilical  veins.  These  are  represented  in  the  belly- 
stalk  by  a  single  venous  trunk  which,  when  it  reaches  the  body  of 
the  embryo,  divides  into  two  stems  which  pass  forward,  one  on  each 
side  of  the  umbilicus,  and  thence  on  each  side  of  the  median  line  of 
the  ventral  abdominal  wall,  to  form  with  the  corresponding  vitelline 
veins  common  trunks  which  open  into  the  ductus  Cuvieri.  As  the 
liver  develops  it  comes  into  intimate  relation  with  the  vitelline  veins, 
which  receive  numerous  branches  from  its  substance  and,  indeed, 
seem  to  break  up  into  a  network  (Fig.  160,  A)  traversing  the  liver 


DC, 


DC, 


DC 


Vus 


Vom.s 


Vud. 


DC 


D.K4 


^drus 


Vorn.s. 


Kl/J. 


Vamd.        Vb.7ns. 


-Diagrams  Illustrating  the  Transformations  of  the  Vitelline  and 
Umbilical  Veins. 

D.C,  Ductus  Cuvieri;  D.V.A,  ductus  venosus;   V.o.m.d  and  V.o.m.s,  right  and  left 
vitelline  veins;  V.u.d  and  V.u.s,  right  and  left  umbilical  veins. — {Hochstetter.) 


substance  and  uniting  again  to  form  two  stems  which  represent  the 
original  continuations  of  the  vitellines.  From  the  point  where  the 
common  trunk  formed  by  the  right  vitelline  and  umbilical  veins 
opens  into  the  Cuvierian  duct  a  new  vein  develops,  passing  down- 
ward and  to  the  left  to  unite  with  the  left  vitelline;  this  is  the  ductus 
venosus  (Fig.  160, B, D.V.A).  In  the  meantime  three  cross-connec- 
tions have  developed  between  the  two  vitelline  veins,  two  of  which 
pass  ventral  and  the  other  dorsal  to  the  intestine,  so  that  the  latter  is 


DEVELOPMENT    OF   THE    VENOUS    SYSTEM 


26l 


surrounded  by  two  venous  loops  (Fig.  161,  A),  and  a  connection  is 
developed  between  each  umbilical  vein  and  the  corresponding 
vitelline  (Fig.  160,  B),  that  of  the  left  side  being  the  larger  and  uniting 
with  the  vitelline  just  where  it  is  joined  by  the  ductus  venosus  so  as 
to  seem  to  be  the  continuation  of  this  vessel  (Fig.  160,  C).  When 
these  connections  are  complete,  the  upper  portions  of  the  umbilical 
veins  degenerate  (Fig.  161),  and  now  the  right  side  of  the  lower  of  the 
two  vitelline  loops  which  surround  the  intestine  disappears,  as  does 
also  that  portion  of  the  left  side  of  the  upper  loop  which  intervenes 


Fig.  161. — A,  The  Venous  Trunks  of  an  Embryo  of  5  mm.  seen  from  the 
Ventral  Surface;  B,  Diagram  Illustrating  the  Transformation  to  the  Adult 
Condition. 

Vcd  and  Vcs,  Right  and  left  superior  venae  cavae;  Vj,  jugular  vein;  V.om,  vitelline 
vein;  Vp,  vena  porta;  Vu,  umbilical  vein  (lower  part);  Vu',  umbilical  vein  (upper 
part);  Vud  and  Vus,  right  and  left  umbilical  veins  (lower  parts). — (His.) 

between  the  middle  cross-connection  and  the  ductus  venosus,  and 
so  there  is  formed  from  the  vitelline  veins  the  vena  porta. 

While  these  changes  have  been  progressing  the  right  umbilical 
vein,  originally  the  larger  of  the  two  (Fig.  160,  A  and  B,  V.u.d), 
has  become  very  much  reduced  in  size  and,  losing  its  connection 
with  the  left  vein  at  the  umbilicus,  forms  a  vein  of  the  ventral  abdom- 
inal wall  in  which  the  blood  now  flows  from  above  downward.     The 


262 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM 


left  umbilical  now  forms  the  only  route  for  the  return  of  blood  from 
the  placenta,  and  appears  to  be  the  direct  continuation  of  the  ductus 
venosus  (Fig.  161,  C),  into  which  open  the  hepatic  veins,  returning 
the  blood  distributed  by  the  portal  vein  to  the  substance  of  the  liver. 
Returning  now  to  the  posterior  cardinal  veins,  it  has  been  found 
that  in  the  rabbit  the  branches  which  come  to  them  from  the  mesen- 
tery anastomose  longitudinally  to  form  a  vessel  lying  parallel  and 
slightly  ventral  to  each  cardinal.     These  may  be  termed  the  sub- 


A  £ 

Fig.  162. — Diagrams  Illustrating  the  Development  or  the  Inferior  Vena  Cava. 
The  cardinal  veins  and  ductus  venosus  are  black,  the  subcardinal  system  blue, 
and  the  supracardinal  yellow,  cs,  coronary  sinus;  dv,  ductus  venosus;  il,  iliac  vein; 
r,  renal;  s,  internal  spermatic;  scl,  subclavian;  sr,  suprarenal;  va,  azygos;  vha,  hemi- 
azygos; vi,  innominate;  vj,  internal  jugular. 


cardinal  veins  (Lewis),  and  in  their  earliest  condition  they  open  at 
either  end  into  the  corresponding  cardinal,  with  which  they  are  also 
united  by  numerous  cross-branches.  Later,  in  rabbits  of  8.8  mm., 
these  cross-branches  begin  to  disappear  and  give  place  to  a  large 
cross-branch  situated  immediately  below  the  origin  of  the  superior 


DEVELOPMENT    OF   THE    VENOUS    SYSTEM  263 

mesenteric  artery,  and  at  the  same  point  a  cross-branch  between  the 
two  subcardinals  also  develops.  The  portion  of  the  right  subcardi- 
al which  is  anterior  to  the  cross-connection  now  rapidly  enlarges 
and  unites  with  the  ductus  venosus  about  where  the  hepatic  veins 
open  into  that  vessel  (Fig.  162,  A),  and  the  portion  of  each  posterior 
cardinal  immediately  above  the  entrance  of  the  renal  veins  degen- 
erates, so  that  all  the  blood  received  by  the  posterior  portions  of  the 
cardinals  is  returned  to  the  heart  by  way  of  the  right  subcardinal, 
its  cross-connections,  and  the  upper  part  of  the  ductus  venosus. 

When  this  is  accomplished  the  lower  portions  of  the  subcardinals 
disappear,  while  the  portions  above  the  large  cross-connection  per- 
sist, greatly  diminished  in  size,  as  the  suprarenal  veins  (Fig.  162,  B). 

In  the  early  stages  the  veins  which  drain  the  posterior  abdominal 
walls  empty  into  the  posterior  cardinals,  and  later  they  form,  in  the 
region  of  the  kidney  on  each  side,  a  longitudinal  anastomosis  which 
opens  at  either  extremity  into  the  posterior  cardinal.  The  ureter 
thus  becomes  surrounded  by  a  venous  ring,  the  dorsal  limb  of  which 
is  formed  by  the  new  longitudinal  anastomosis,  which  has  been 
termed  the  supracardinal  vein  (McClure  and  Huntington),  while  the 
ventral  limb  is  formed  by  a  portion  of  the  posterior  cardinal  (Fig. 
162,  B).  Still  later  the  ventral  limb  of  the  loop  disappears  and  the 
dorsal  supracardinal  limb  replaces  a  portion  of  the  more  primitive 
posterior  cardinal.  An  anastomosis  now  develops  between  the 
right  and  left  cardinals  at  the  point  where  the  iliac  veins  open  into 
them  (Fig.  162,  B),  and  the  portion  of  the  left  cardinal  which  inter- 
venes between  this  anastomosis  and  the  entrance  of  the  internal 
spermatic  vein  disappears,  the  remainder  of  it,  as  far  forward  as  the 
renal  vein,  persisting  as  the  upper  part  of  the  left  internal  spermatic 
vein,  which  thus  comes  to  open  into  the  renal  vein  instead  of  into 
the  vena  cava  as  does  the  corresponding  vein  of  the  right  side  of  the 
body  (Fig.  162,  C,  s).  The  renal  veins  originally  open  into  the 
cardinals  at  the  point  where  these  are  joined  by  the  large  cross- 
connection,  and  when  the  lower  part  of  the  left  cardinal  disappears, 
this  cross-connection  forms  the  proximal  part  of  the  left  renal  vein, 
which  consequently  receives  the  left  suprarenal  (Fig.  162,  C). 


264  DEVELOPMENT    OF    THE    VENOUS    SYSTEM 

The  observations  upon  which  the  above  description  is  based 
have  been  made  upon  the  rabbit,  but  it  seems  probable  from  the 
partial  observations  that  have  been  made  that  similar  changes 
occur  also  in  the  human  embryo.  It  will  be  noted  from  what  has 
been  said  that  the  inferior  vena  cava  is  a  composite  vessel,  consisting 
of  at  least  four  elements:  (1)  the  proximal  part  of  the  ductus  venosus; 
(2)  the  anterior  part  of  the  right  subcardinal;  (3)  the  right  supra- 
cardinal;  and  (4)  the  posterior  part  of  the  right  cardinal. 

The  complicated  development  of  the  inferior  vena  cava  naturally 
gives  rise  to  numerous  anomalies  of  the  vein  due  to  inhibitions  of  its 
development.  These  anomalies  affect  especially  the  post-renal  portion,  a 
persistence  of  both  cardinals  (interpreting  the  conditions  in  the  terms  of 
what  occurs  in  the  rabbit)  giving  rise  to  a  double  post-renal  cava,  or  a 
persistence  of  the  left  cardinal  and  the  disappearance  of  the  right  to  a 
vena  cava  situated  on  the  left  side  of  the  vertebral  column  and  crossing 
to  the  right  by  way  of  the  left  renal  vein.  So,  too,  the  occurrence  of 
accessory  renal  veins  passing  dorsal  to  the  ureter  is  explicable  on  the 
supposition  that  they  represent  portions  of  the  supracardinal  system  of 
veins. 

It  has  already  been  noted  that  the  portions  of  the  posterior 
cardinals  immediately  anterior  to  the  entrance  of  the  renal  veins 
disappear.  The  upper  part  of  the  right  vein  persists,  however,  and 
becomes  the  vena  azygos  of  the  adult,  while  the  upper  portion  of  the 
left  vein  sends  a  cross-branch  over  to  unite  with  the  azygos  and  then 
separates  from  the  coronary  sinus  to  form  the  vena  hemiazygos.  At 
least  this  is  what  is  described  as  occurring  in  the  rabbit.  In  the  cat, 
however,  only  the  very  uppermost  portion  of  the  right  posterior 
cardinal  persists  and  the  greater  portion  of  the  azygos  and  perhaps 
the  entire  hemiazygos  vein  is  formed  from  the  prerenal  portions  of 
the  supracardinal  veins,  the  right  one  joining  on  to  the  small  per- 
sisting upper  portion  of  the  right  posterior  cardinal,  while  the  cross- 
connection  between  the  hemiazygos  and  azygos  represents  one  of  the 
originally  numerous  cross-connections  between  the  supracardinals. 

The  ascending  lumbar  veins,  frequently  described  as  the  commence- 
ments of  the  azygos  veins,  are  in  reality  secondary  formations  developed 
by  the  anastomoses  of  anteriorly  and  posteriorly  directed  branches  of  the 
lumbar  veins, 


DEVELOPMENT    OF   THE    VENOUS    SYSTEM  26 


The  Development  of  the  veins  of  the  Limbs. — The  development  of 
the  limb  veins  of  the  human  embryo  requires  further  investigation, 
but  from  a  comparison  of  what  is  known  with  what  has  been  observed 
in  rabbit  embryos  it  may  be  presumed  that  the  changes  which  take 
place  are  somewhat  as  follows :  In  the  anterior  extremity  the  blood 
brought  to  the  limb  is  collected  by  a  vein  which  passes  distally  along 
the  radial  border  of  the  limb  bud,  around  its  distal  border,  and  prox- 
imally  along  its  ulnar  border  to  open  into  the  anterior  cardinal  vein; 
this  is  the  primary  ulnar  vein.  Later  a  second  vein  grows  out  from 
the  external  jugular  along  the  radial  border  of  the  limb,  representing 
the  cephalic  vein  of  the  adult,  and  on  its  appearance  the  digital  veins, 
which  were  formed  from  the  primary  ulnar  vein,  become  connected 
with  it,  and  the  distal  portion  of  the  primary  ulnar  vein  disappears. 
Its  proximal  portion  persists,  however,  to  form  the  basilic  vein,  from 
which  the  brachial  vein  and  its  continuation,  the  ulnar  vein,  are 
developed,  while  the  radial  vein  develops  as  an  outgrowth  from  the 
cephalic,  which  at  an  early  stage  secures  an  opening  into  the  axillary 
vein,  its  original  communication  with  the  external  jugular  forming 
the  jugulo-cephalic  vein. 

In  the  lower  limb  a  primary  fibular  vein,  exactly  comparable  to 
the  primary  ulnar  of  the  arm,  surrounds  the  distal  border  of  the  limb- 
bud  and  passes  up  its  fibular  border  to  open  with  the  posterior 
cardinal  vein.  The  further  development  in  the  lower  limb  differs 
considerably,  however,  from  that  of  the  upper  limb.  From  the  pri- 
mary fibular  vein  an  anterior  tibial  vein  grows  out,  which  receives 
the  digital  branches  from  the  toes,  and  from  the  posterior  cardinal, 
anterior  to  the  point  where  the  primary  fibular  opens  into  it,  a  vein 
grows  down  the  tibial  side  of  the  leg,  forming  the  long  saphenous  vein. 
From  this  the  femoralvein  is  formed  and  from  it  the  posterior  tibial 
vein  is  continued  down  the  leg.  An  anastomosis  is  formed  between 
the  femoral  and  the  primary  fibular  veins  at  the  level  of  the  knee  and 
the  proximal  portion  of  the  latter  vein  then  becomes  greatly  reduced, 
while  its  distal  portion  possibly  persists  as  the  small  saphenous  vein 
(Hochstetter). 

The  Pulmonary  Veins. — The  development  of  the  pulmonary  veins 


266 


THE    FETAL    CIRCULATION 


has  already  been  described  in  connection  with  the  development  of 
the  heart  (see  p.  234). 

The  Fetal  Circulation. — During  fetal  life  while  the  placenta  is 
the  sole  organ  in  which  occur  the  changes  in  the  blood  on  which  the 


Fig.  163. — The  Fetal  Circulation. 
ao,  Aorta;  a.pu.,  pulmonary  artery;    au,  umbilical  artery;    da,  ductus  arteriosus; 
dv,  ductus  venosus;  int,  intestine;  vci  and  vcs,  inferior  and  superior  vena  cava;  vh, 
hepatic   vein;   vp,   vena  portas;   v.pu,   pulmonary   vein;  vu,   umbilical  vein. — {From 
Kollmann.) 


nutrition  of  the  embryo  depends,  the  course  of  the  blood  is  neces- 
sarily somewhat  different  from  what  obtains  in  the  child  after  birth. 
Taking  the  placenta  as  the  starting-point,  the  blood  passes  along  the 


THE    FETAL    CIRCULATION  267 

umbilical  vein  to  enter  the  body  of  the  fetus  at  the  umbilicus,  whence 
it  passes  forward  in  the  free  edge  of  the  ventral  mesentery  (see  p.  321) 
until  it  reaches  the  liver.  Here,  owing  to  the  anastomoses  between 
the  umbilical  and  vitelline  veins,  a  portion  of  the  blood  traverses  the 
substance  of  the  liver  to  open  by  the  hepatic  veins  into  the  inferior 
vena  cava,  while  the  remainder  passes  on  through  the  ductus  venosus 
to  the  cava,  the  united  streams  opening  into  the  right  atrium.  This 
blood,  whose  purity  is  only  slightly  reduced  by  mixture  with  the 
blood  returning  from  the  inferior  vena  cava,  is  prevented  from  pass- 
ing into  the  right  ventricle  by  the  Eustachian  valve,  which  directs  it 
to  the  foramen  ovale,  and  through  this  it  passes  into  the  left  atrium, 
thence  to  the  left  ventricle,  and  so  out  by  the  systemic  aorta. 

The  blood  which  has  been  sent  to  the  head,  neck,  and  upper 
extremities  is  returned  by  the  superior  vena  cava  also  into  the  right 
atrium,  but  this  descending  stream  opens  into  the  atrium  to  the  right 
of  the  annulus  of  Vieussens  (see  Fig.  141)  and  passes  directly  to  the 
right  ventricle  without  mingling  to  any  great  extent  with  the  blood 
returning  by  way  of  the  inferior  cava.  From  the  right  ventricle 
this  blood  passes  out  by  the  pulmonary  artery;  but  the  lungs  at  this 
period  are  collapsed  and  in  no  condition  to  receive  any  great  amount 
of  blood,  and  so  the  stream  passes  by  way  of  the  ductus  arteriosus  into 
the  systemic  aorta,  meeting  there  the  placental  blood  just  below  the 
point  where  the  left  subclavian  artery  is  given  off.  From  this  point 
onward  the  aorta  contains  only  mixed  blood,  and  this  is  distributed 
to  the  walls  of  the  thorax  and  abdomen  and  to  the  lungs  and  abdom- 
inal viscera,  the  greater  part  of  it,  however,  passing  off  in  the  hypo- 
gastric arteries  and  so  out  again  to  the  placenta. 

This  is  the  generally  accepted  account  of  the  fetal  circulation  and  it 
is  based  upon  the  idea  that  the  foramen  ovale  is  practically  a  connection 
between  the  inferior  vena  cava  and  the  left  atrium.  If  it  be  correct  the 
right  ventricle  receives  only  the  blood  returning  to  the  heart  by  the  vena 
cava  superior,  while  the  left  receives  all  that  returns  by  the  inferior  vena 
cava  together  with  what  returns  by  the  pulmonary  veins.  One  would, 
therefore,  expect  that  the  capacity  and  pressure  of  the  right  ventricle 
would  in  the  fetus  be  less  than  those  of  the  left.  Pohlman,  who  has 
recently  investigated  the  question  in  embryo  pigs,  finds,  on  the  contrary, 
that  the  capacities  and  pressures  of   the  two  ventricles  are  equal  and 


268  DEVELOPMENT    OF    THE    LYMPHATIC    SYSTEM 

maintains  that  the  foramen  ovale  is  actually  a  connection  between  the 
two  atria.  That  is  to  say,  he  holds  that  there  is  an  actual  mingling  of  the 
blood  from  the  two  venae  cava?  in  the  right  atrium,  whence  the  mixed 
blood  passes  to  the  right  ventricle,  a  certain  amount  of  it,  however, 
passing  through  the  foramen  ovale  and  so  to  the  left  ventricle  to  equalize 
the  deficiency  that  would  otherwise  exist  in  that  chamber  owing  to  the 
small  amount  of  blood  returning  by  the  pulmonary  veins.  According 
to  this  view  there  would  be  no  difference  in  the  quality  of  the  blood  distri- 
buted to  different  portions  of  the  body,  such  as  is  provided  for  by  the 
current  theory;  all  the  blood  leaving  the  heart  would  be  mixed  blood  and 
in  favor  of  this  view  is  the  fact  that  starch  granules  injected  into  either 
the  superior  or  the  inferior  vena  cava  in  living  pig  embryos  were  in  all 
cases  recovered  from  both  sides  of  the  heart. 

At  birth  the  lungs  at  once  assume  their  functions,  and  on  the 
cutting  of  the  umbilical  cord  all  communication  with  the  placenta 
ceases.  Shortly  after  birth  the  foramen  ovale  closes  more  or  less 
perfectly,  and  the  ductus  arteriosus  diminishes  in  size  as  the  pul- 
monary arteries  increase  and  becomes  eventually  converted  into  a 
fibrous  cord.  The  hypogastric  arteries  diminish  greatly,  and  after 
they  have  passed  the  bladder  are  also  reduced  to  fibrous  cords,  a  fate 
likewise  shared  by  the  umbilical  vein,  which  becomes  converted 
into  the  round  ligament  of  the  liver. 

The  Development  of  the  Lymphatic  System. — Concerning 
the  development  of  the  lymphatic  system  two  discordant  views 
exist,  one  (Sabin,  Lewis)  regarding  it  in  its  entirety  as  a  direct 
development  from  the  venous  system,  while  the  other  (Huntington, 
McClure)  recognizes  for  it  a  dual  origin,  a  portion  being  derived 
directly  from  the  venous  system  and  the  rest  from  a  series  of  mesen- 
chymal spaces  developing  in  relation  to  veins  but  quite  unconnected 
with  them. 

The  portion  of  the  system  concerning  which  harmony  prevails 
is  that  which  forms  the  connection  with  the  venous  system  in  the 
adult  and  constitutes  what  in  the  embryo  is  termed  the  jugular 
lymph  sac.  In  the  early  stages  of  development  a  capillary  network 
extends  along  the  line  of  the  jugular  veins,  communicating  with 
them  at  various  points.  In  embryos  of  10  mm.  a  portion  of  this 
network,  on  either  side  of  the  body,  becomes  completely  separated 


bEVELOPMENT    OF   THE    LYMPHATIC    SYSTEM 


269 


from  the  jugular  and  gives  rise  to  a  number  of  closed  cavities  lined 
with  endothelium  and  situated  in  the  neighborhood  of  the  junction 
of  the  primary  ulnar  and  cephalic  veins  with   the   jugular.     In 


Fig.  164. — Diagrams  showing  the  Arrangement  of  the  Lymphatic  Vessels  in 
Pig  Embryos  of  (4)  20  mm.  and  (B)  40  mm. 
ACV,  Jugular  vein;  ADR,  suprarenal  body;  ALU,  jugular  lymph  sac;  Ao,  aorta 
Arm  D,  deep  lymphatics  to  the  arm;  D,  diaphragm;  Du,  branches  to  duodenum 
FV,  femoral  vein;  H,  branches  to  heart;  K,  kidney;  LegD,  deep  lymphatics  to  leg 
Lu,  branches  to  lung;  MP,  branches  to  mesenteric  plexus;  CE,  branch  to  oesophagus 
PCV,   cardinal   vein:   PLH,   posterior   lymph   sac;  RC,   cisterna   chyli;  RLD,   right 
lymphatic  duct;  ScV,  subclavian  vein;  SV,  sciatic  vein;  St,  branches  to  stomach;  TD, 
thoracic  duct;  WB.  Wolffian  body. — (Sdbin.) 

later  stages  these  cavities  enlarge  and  unite  to  form  a  large  sac,  the 
jugular  lymph  sac  (Fig.  164,  ALU),  and  this,  still  later,  makes  a 


270  DEVELOPMENT   OF   THE    LYMPHATIC   SYSTEM 

new  connection  with  the  jugular,  the  opening  being  guarded  by  a 
valve.  This  communication  becomes  the  adult  communication  of 
the  thoracic  duct  or  right  lymphatic  duct  with  the  venous  system, 
but  the  sac  itself,  as  development  proceeds,  becomes  divided  into 
smaller  portions  and  gives  rise  to  a  number  of  lymph  nodes. 

A  similar  pair  of  lymph  sacs  also  develop  in  relation  to  the 
sciatic  vein,  but  their  exact  mode  of  origin  is  uncertain.  In  embryos 
of  20  mm.  venous  plexuses,  similar  to  the  jugular  plexuses  of 
younger  stages,  are  found  accompanying  the  sciatic  veins,  and  a 
little  later  there  are  found  in  the  same  region  a  pair  of  posterior  or 
sciatic  lymph  sacs  (Fig.  164,  PLH),  which,  like  the  jugular  sacs, 
later  give  rise  to  a  series  of  lymph  nodes.  At  about  the  same  stage 
of  development  &  retroperitoneal  sac  (Fig.  165,  Lsr)  is  also  formed  in 
the  root  of  the  mesentery  cranial  to  the  origin  of  the  superior  mesen- 
teric artery,  and  this,  too,  later  gives  rise  to  a  plexus  of  lymphatic 
vessels  in  connection  with  which  the  mesenteric  lymphatic  nodes 
develop.  This  last  sac  is  much  more  pronounced  in  the  pig  embryo 
than  in  man,  and  in  that  form  it  has  been  found  to  have  its  origin 
from  a  capillary  network  that  separates  from  the  renal  veins 
(Baetjer). 

There  are  thus  formed  five  sacs,  all  of  which  are  associated  with 
the  formation  of  groups  of  lymphatic  nodes,  and  in  the  case  of  one 
pair  at  least  it  is  agreed  that  they  are  directly  developed  from  venous 
capillaries.  It  is  in  connection  with  the  remaining  sac  and  espe- 
cially with  the  formation  of  the  thoracic  duct  and  the  peripheral 
lymphatics  that  the  want  of  harmony  referred  to  above  occurs. 
The  first  portion  of  the  thoracic  duct  to  appear  is  the  cisterna  chyli, 
which  is  found  in  embryos  of  23  mm.  in  the  region  of  the  third  and 
fourth  lumbar  segments,  in  close  proximity  to  the  vena  cava  (Fig. 
165,  Cc).  After  its  appearance  the  rest  of  the  thoracic  duct  develops 
quickly,  it  being  completely  formed  in  embryos  of  30  mm.,  and  it  is 
interesting  to  note  that  at  this  stage  the  duct  is  paired  in  its  caudal 
portion,  two  trunks  passing  forward  from  the  cisterna  chyli,  the 
right  one  passing  behind  the  aorta  and  uniting  with  the  left  after  it 
has  entered  the  thorax. 


DEVELOPMENT    OF    THE    LYMPHATIC    SYSTEM 


271 


The  mode  of  origin  of  the  duct  has  not  yet  been  made  out  in 
human  embryos.  In  the  pig  and  rabbit  isolated  spaces  lined  with 
endothelium  occur  along  the  course  of  the  duct,  but  without  com- 
municating with  it,  and  the  fact  that  some  of  these  showed  connec- 
tion with  the  neighboring  azygos  veins  gave  basis  for  the  view  that 
they  were  the  remains  of  a  venous  capillary  plexus  from  which  the 
duct  had  developed.     It  is  possible,  however,  that  the  duct  is  formed 


T  Fig.  165. — Diagram  of  the  Posterior  Portion  of  the  Body  of  a  Human 
Embryo  of  23  mm.,  showing  the  Relations  of  the  Retroperitoneal  Lymph 
Sac  and  the  Cisterna  Chyli  to  the  Veins. 

Am,  Superior  mesenteric  artery;  Ao,  aorta;  Cc,  cisterna  chyli;  Isr,  retroperitoneal 
lymph  sac;  S,  suprarenal  body;  Va,  vena  azygos;  Vci,  vena  cava  inferior;  vlu  first 
lumbar  vertebra;  vsu  first  sacral  vertebra. — (After  Sabin.) 

by  the  union  of  outgrowths  from  the  cisterna  chyli  and  jugular  sac, 
in  which  case  it  would  also  be  a  derivative  of  the  venous  system, 
provided  that  the  cisterna  chyli  is  formed  in  the  same  way  as  the 
jugular  sac.     Huntington  and  McClure,  however,  maintain  that  it 


272 


DEVELOPMENT   OF   THE    LYMPHATIC   SYSTEM 


is  formed  by  the  fusion  of  spaces  appearing  in  the  mesenchyme 
immediately  external  to  the  intima  of  degenerating  veins;  hence  the 
spaces  are  termed  extraintimal  spaces.  These  at  first  have  no 
endothelial  lining  and  they  are  never  in  connection  with  the  lumina 
of  the  veins.  They  are  perfectly  independent  structures  and  any 
connections  they  may«nake  with  the  venous  system  are  entirely 
secondary.     This  mode  of  origin  from  extraintimal  spaces  is  not 

confined  to  the  thoracic  duct,  according 
to  the  authors  mentioned,  but  is  the 
method  of  development  of  all  parts  of 
the  lymphatic  system,  with  the  exception 
of  the  jugular  sacs.  According  to  the 
supporters  of  the  direct  venous  origin 
the  peripheral  lymphatic  stems  develop, 
like  blood-vessels,  as  outgrowths  from 
the  stems  already  present. 

Lymph  nodes  nave  not  been  observed 
in  human  embryos  until  toward  the  end 
of  the  third  month  of  development,  but 
'  !.<l'-V''\LY.  they  appear  in  pig  embryos  of  3  cm. 
X^Hi^.  Their  unit  of  structure  is  a  blood-vessel, 
breaking  up  at  its  termination  into  a 
leash  of  capillaries,  around  which  a  con- 
densation of  lymphocytes  occurs  in  the 
mesenchyme.  A  structure  of  this  kind 
forms  what  is  termed  a  lymphoid  follicle 
and  may  exist,  even  in  this  simple  condition,  in  the  adult.  More 
frequently,  however,  there  are  associated  with  the  follicle  lymphatic 
vessels,  or  rather  the  follicle  develops  in  a  network  of  lymphatic 
vessels,  which,  become  an  investment  of  the  follicle  and  form  with  it  a 
simple  lymph  node  (Fig.  166).  This  condition  is,  however,  in  many 
cases  but  transitory,  the  artery  branching  and  collections  of  lym- 
phoid tissue  forming  around  each  of  the  branches,  so  that  a  series 
of  follicles  are  formed,  which,  together  with  the  surrounding  lym- 
phatic vessels,  become  enclosed  by  a  connective-tissue  capsule  to 


Fig.  166.— Diagram  of  a 
Primary  Lymph  Node  of  an 
Embryo  Pig  of  8  cm. 
a,  Artery;  aid,  afferent  lymph 
duct;  eld,  efferent  lymph  duct; 
/,  follicle. — (Sabin.) 


DEVELOPMENT    OF    THE    LYMPHATIC    SYSTEM 


273 


form  a  compound  lymph  node.  Later  trabecular  of  connective  tissue 
extend  from  the  capsule  toward  the  center  of  the  node,  between  the 
follicles,  the  lymphatic  network  gives  rise  to  peripheral  and  central 
lymph  sinuses,  and  the  follicles,  each  with  its  arterial  branch,  con- 
stitute the  peripheral  nodules  and  the  medullary  cords,  the  portions 
of  these  immediately  surrounding  the  leash  of  capillaries  into  which 


tt-  be 


Fig.  167. — Developing  H^emolymph  Node. 

be,  central  blood-vessel;  bh,  blood-vessel  at  hilus;  ps,  peripheral  blood  sinus. — (Sabin 

from  Morris'  Human  Anatomy.) 


the  artery  dissolves,  constituting  the  so-called  germ  centers  in  which 
multiplication  of  the  lymphocytes  occurs. 

In  various  portions  of  the  body,  but  especially  along  the  root  of 

the  mesentery,  what  are  termed  hcemolymph  nodes  occur.     In  these 

the  lymph  sinus  is  replaced  by  a  blood  sinus,  but  with  this  exception 

their  structure  resembles  that  of  an  ordinary  lymph  node,  a  simple 

18 


274  DEVELOPMENT    OF   THE    SPLEEN 

one  consisting  of  a  follicle,  composed  of  adenoid  tissue  with  a  central 
blood-vessel,  and  a  peripheral  blood  sinus  (Fig.  167). 

The  Development  of  the  Spleen. — Recent  studies  (Mall)  have 
shown  that  the  spleen  may  well  be  regarded  as  possessing  a  structure 
comparable  to  that  of  the  lymph  nodes,  the  pulp  being  more  or  less 
distinctly  divided  by  trabecular  into  areas  termed  pulp  cords,  the  axis 
of  each  of  which  is  occupied  by  a  twig  of  the  splenic  artery.  The 
spleen,  therefore,  seems  to  fall  into  the  same  category  of  organs  as 
the  lymph  and  hsemolymph  nodes,  differing  from  these  chiefly  in 
the  absence  of  sinuses.  It  has  generally  been  regarded  as  a  develop- 
ment of  the  mesenchyme  situated  between  the  two  layers  of  the 
mesogastrium.  To  this  view,  however,  recent  observers  have 
taken  exception,  holding  that  the  ultimate  origin  of  the  organ  is  in 
part  or  entirely  from  the  ccelomic  epithelium  of  the  left  layer  of  the 
mesogastrium.  The  first  indication  of  the  spleen  has  been  observed 
in  embryos  of  the  fifth  week  as  a  slight  elevation  on  the  left  (dorsal) 
surface  of  the  mesogastrium,  due  to  a  local  thickening  and  vasculariza- 
tion of  the  mesenchyme,  accompanied  by  a  thickening  of  the 
ccelomic  epithelium  which  covers  the  elevation.  The  mesenchyme 
thickening  presents  no  differences  from  the  neighboring  mesenchyme, 
but  the  epithelium  is  not  distinctly  separated  from  it  over  its  entire 
surface,  as  it  is  elsewhere  in  the  mesentery.  In  later  stages,  which 
have  been  observed  in  detail  in  pig  and  other  amniote  embryos, 
cells  separate  from  the  deeper  layers  of  the  epithelium  (Fig.  168)  and 
pass  into  the  mesenchyme  thickening,  whose  tissue  soon  assumes  a 
different  appearance  from  the  surrounding  mesenchyme  by  its  cells 
being  much  crowded.  This  migration  soon' Ceases,  however,  and 
in  embryos  of  forty-two  days  the  ccelomic  epithelium  covering  the 
thickening  is  reduced  to  a  simple  layer  of  cells. 

The  later  stages  of  development  consist  of  an  enlargement  of 
the  thickening  and  its  gradual  constriction  from  the  surface  of  the 
mesogastrium,  until  it  is  finally  united  to  it  only  by  a  narrow  band 
through  which  the  large  splenic  vessels  gain  access  to  the  organ 
The  cells  differentiate  themselves  into  trabecular  and  pulp   cords 


DEVELOPMENT    OF    THE    SPLEEN 


7d 


special  collections  of  lymphoid  cells  around  the  branches  of   the 
splenic  artery  forming  the  Malpighian  corpuscles. 

It  has  already  been  pointed  out  (p.  225)  that  during  embryonic  life 
the  spleen  is  an  important  haematopoietic  organ,  both  red  and  white 
corpuscles  undergoing  active  formation  within  its  substance.  The 
Malpighian  corpuscles  are  collections  of  lymphocytes  in  which  multipli- 
cation takes  place,  and  while  nothing  is  as  yet  known  as  to  the  fate  of  the 
cells  which  are  contributed  to  the  spleen  from  the  ccelomic  epithelium, 
since  they  quickly  come  to  resemble  the  mesenchyme  cells  with  which 
they  are  associated,  yet  the  growing  number  of  observations  indicating 
an  epithelial  origin  for  lymphocytes  suggests  the  possibility  that  the  cells 
in  question  may  be  responsible  for  the  first  leukocytes  of  the  spleen. 


"      '      .      .  - 


ms 


Fig.  168. — Section  through  the  Left  Layer  of  the  Mesogastrium  of  a  Chick 

Embryo  of  Ninety-three  Hours,  Showing  the  Origin  of  the  Spleen. 

ep,  Ccelomic  epithelium;  ms,  mesenchyme. — {Tonkoff.) 

The  Coccygeal  or  Luschka's  Ganglion. — In  embryos  of  about  15 
cm.  there  is  to  be  found  on  the  ventral  surface  of  the  apex  of  the 
coccyx  a  small  oval  group  of  polygonal  cells,  clearly  separated  from 
the  surrounding  tissue  by  a  mesenchymal  capsule.  Later,  connec- 
tive-tissue trabecular  make  their  way  into  the  mass,  which  thus 
becomes  divided  into  lobules,  and,  at  the  same  time,  a  rich  vascular 
supply,  derived  principally  from  branches  of  the  middle  sacral  artery, 
penetrates  the  body,  which  thus  assumes  the  adult  condition  in 
which  it  presents  a  general  resemblance  to  a  group  of  lymph  follicles. 

It  has  generally  been  supposed  that  the  coccygeal  ganglion  was  in 
part  derived  from  the  sympathetic  nervous  system  and  belonged  to 
the  same  group  of  organs  as  the  suprarenal  bodies.     The  most  recent 


276  LITERATURE 

work  on  its  development  (Stoerk)  tends,  however,  to  disprove  this 
view,  and  the  ganglion  seems  accordingly  to  find  its  place  among 
the  lymphoid  organs. 

LITERATURE. 

W.  A.  Baetjer:  "On  the  Origin  of  the  Mesenteric  Sac  and  the  Thoracic  Duct  in  the 

Embryo  Pig,"  Amer.  Journ.  Anat.,  vin,  1908. 
E.  van  Beneden  and  C.  Julln:  "Recherches  sur  la  formation  des  annexes  fcetales 

chez  les  mammiferes,"  Archives  de  Biolog.,  v,  1884. 
A.  C.  Bernays:  "  Entwickehingsgeschichte  der  Atrioventricularklappen,"  Morphol. 

Jahrbuch,  11,  1876. 
G.    Born:  "Beitrage   zur   Entwicklungsgeschichte   des   Saugethierherzens,"    Archiv 

fiir  mikrosk.  Anat.,  xxxiii,  1889. 
J.  L.  Bremer:  "  On  the  Origin  of  the  Pulmonary  Arteries  in  Mammals,"  Anat.  Record, 

in,  1909. 
I.  Broman:  "Ueber  die  Entwicklung,  Wanderung  und  Variation  der  Bauchaorten- 

zweige  bei  den  Wirbeltiere,"  Ergeb.  Anat.  und  Entwick.,  xvi,  1906. 
I.  Broman:  "  Ueber  die  Entwicklung  und  "Wanderung"  der  Zweige  der  aorta  abdom- 

inalis  beim  Menschen,"  Anat.  Hefte,  XXXVI,  1908. 
E.  E.  Butterfield:  "Ueber  die  ungranulierte  Vorstufen  der  Myelocyten  und  ihre 

Bildung  in  Milz,  Leber  und  Lymphdriisen,"  Deutsch.  Arch.  f.  klin.  Med.,  xcn, 

1908. 

E.  R.  Clark:  "  Observations  on  Living  Growing  Lymphatics  in  the  Tail  of  the  Frog 

Larva,"  Anat.  Record,  in,  1909. 

C.  B.  Coulter:  "The  Early  Development  of  the  Aortic  Arches  of  the  Cat,  with 

Especial  Reference  to  the  Presence  of  a  Fifth  Arch."  Anat.  Record,  III,  1909. 

D .  M.  Davis:  "  Studies  on  the  Chief  Veins  in  Early  Pig  Embryos  and  the  Origin  of  the 

Vena  Cava  Inferior,"  Amer.  Journ.  Anat.,  x,  1910. 
J.  Disse:  "Die  Entstehung  des  B lutes  und  der  ersten  Gefasse  im  Huhnerei,"  Archiv 

fiir  mikrosk.  Anat.,  xvi,  1879. 
A.  C.  F.  Eternod:  "Premiers  stades  de  la  circulation  sanguine  dans  l'ceuf  et  Pembryon 

humain,"  Anat.  Anzeiger,  xv,  1899. 
H.  M.  Evans:  "On  the  Development  of  the  Aortae,  Cardinal  and  Umbilical  Veins, 

and  the  other  Blood-vessels  of  Vertebrate  Embryos  from  Capillaries,"   Anat. 

Record,  in,  1909. 
V.  Federow:  "Ueber  die  Entwicklung  der  Lungenvene,"  Anat.  Hefte,  xl,  1910. 
W.  Felix:  "  Zur  Entwicklungsgeschichte  der  Rumpfarterien  des  menschlichen  Embryo," 

Morphol.  Jahrb.,  xli,  1910. 
G.  J.  Heuer:  "The  Development  of  the  Lymphatics  in  the  Small  Intestine  of  the 

Pig,"  Amer.  Journ.  Anat.,  ix,  1909. 
W.  His:  "Anatomie  menschlicher  Embryonen,"  Leipzig,  1880-1882. 

F.  Hochstetter:  "Ueber  die  ursprungliche  Hauptschlagader  der  hinteren  Gliedmasse 

des  Menschen  und  der  Saugethiere,  nebst  Bemerkungen  iiber  die  Entwicklung  der 
Endaste  der  Aorta  abdominalis,"  Morphol.  Jahrbuch,  xvi,  1890. 


LITERATURE  277 

F.  Hochstetter:  "Ueber  die  Entwicklung  der  A.  vertebralis  beim  Kaninchen,  nebst 
Bemerkungen  uber  die  Entstehung  der  Ansa  Vieusseni,"  Morphol.  Jahrbuch,  XVI, 
1890. 

F.  Hochstetter:    "Beitrage    zur    Entwicklungsgeschichte    des   Venensystems    der 

Amnioten."  Morphol.  Jahrbuch,  xx,  1893. 
W.  H.  Howell:  "The  Life-history  of  the  Formed  Elements  of  the  Blood,  Especially 

the  Red  Blood-corpuscles,"  Journ.  of  Morphol.,  iv,  1890. 
W.  H.  Howell:  "Observations  on  the  Occurrence,  Structure,  and  Function  of  the 

Giant-cells  of  the  Marrow,"  Journ.  of  M  or  ph.,  rv,  1890. 

G.  S.  Huntington:  "The  Genetic  Principles  of  the  Development  of  the  Systemic 

Lymphatic  Vessels  in  the  Mammalian  Embryo,"  Anal.  Record,  iv,  1910. 
G.  S.  Huntington:  "The  Anatomy  and  Development  of  the  Systemic  Lymphatic 

Vessels  of  the  Domestic  Cat,"  Memoirs  of  Wistar  Institute,  1,  1912. 
G.  S.  Huntington  and  C.  F.  W.  McClure:  "Development  of  Post-cava  and  Tribu- 
taries in  the  Domestic  Cat,"  Amer.  Journ.  Anat.,  vi,  1907. 
G.  S.  Huntington  and  C.  F.  W.  McClure:  "The  Development  of  the  Main  Lymph 

Channels  of  the  Cat  in  their  Relations  to  the  Venous  System,"  Amer.  Journ 

Anat.,  vi,  1907. 
G.  S.  Huntington  and  C.  F.  W.  MtjClure:  "The  Anatomy  and  Development  of 

the  Jugular  Lymph  Sacs  in  the  Domestic  Cat,"  Amer.  Journ.  Anat.,  x,  1910. 
H.  E.  Jordan:  "A  Microscopical  Study  of  the  Umbilical  Vesical  of  a  13  mm.  Human 

Embryo,  with  Special  Reference  to  the  Entodermal  Tubules    and    the  Blood 

Islands,"  Anat.  Anzeiger,  xxxvn,  1910. 
C.  A.  Kling:  "Studien  uber  die  Entwicklung  der  Lymphdriisen  beim  Menschen," 

Archiv.fiir  mikrosk.  Anal.,  lxiii,  1904. 
H.  Lehmann:  "  On  the  Embryonic  History  of  the  Aortic  Arches  in  Mammals,"  Anat. 

Anzeiger,  xxvi,  1905. 
F.  T.  Lewis:  "The  Development  of  the  Vena  Cava  Inferior,"  Amer.  Journ.  of  Anat., 

1,  1902. 
F.  T.  Lewis:  "The  Development  of  the  Veins  in  the  Limbs  of  Rabbit  Embryos." 

Amer.  Journ.  Anat.,  v,  1906. 
F.  T.  Lewis:  "The  Development  of  the  Lymphatic  System  in  Rabbits,"  Amer.  Journ. 

Anat.,  v,  1906. 
F.  T.  Lewis:  "On  the  Cervical  Veins  and  Lymphatics  in  Four  Human  Embryos," 

Amer.  Journ.  Anat.,  ix,  1909. 
F.  T.  Lewis:  "The  First  Lymph  Glands  in  Rabbit  and  Human  Embryos,"  Anat. 

Record,  in,  1909. 
W.  A.  Locy:  "The  Fifth  and  Sixth  Aortic  Arches  in  Chick  Embryos,  with  Comments 

on  the  Condition  of  the  same  Vessels  in  other  Vertebrates,"   Anat.  Anzeiger 

xxix,  1906. 
F.  P.  Mall:  "Development  of  the  Internal  Mammary  and  Deep  Epigastric  Arteries 

in  Man,"  Johns  Hopkins  Hospital  Bulletin,  1898. 
F.  P.  Mall:  "On  the  Developmennt  of  the  Blood-vessels  of  the  Brain  in  the  Human 

Embryo,"  Amer.  Journ.  Anat.,  iv,  1905. 
A.  Maximow:  "  Untersuchungen  liber  Blut  und  Bindegewebe,"  Arch,  fur  mikr.  Anat., 

Lxxni,  1909;  lxxiv,  1909;  lxxvi,  1910. 


278  LITERATURE 

C.  F.  W.  McClure:  "The  Development  of  the  Thoracic  and  Right  Lymphatic  Ducts 

in  the  Domestic  Cat  (Felis  Domestica),"  Anat.  Anzeiger,  xxxii,  1908. 
C.  F.  W.  McClure:  "  The  Extra-intimal  Theory  of  the  Development  of  the  Mesenteric 

Lymphatics  in  the  Domestic  Cat,"  Verhandl.  Anat.  Gesellsch.,  xxiv,  1910. 
C.  S.  Minot:  "On  a  Hitherto  Unrecognized  Form  of  Blood  Circulation  without 

Capillaries  in  the  Organs  of  Vertebrata,"  Proc.  Boston  Soc.  Nat.  Hist.,  xxix,  1900. 
S.  Molleer:  "Die  Blutbildung  in  der  Embryonalen  Leber  des  Menschen  und  der 

Saugetiere,"  Arch.filr  mikrosk.  Anat.,  Lxxrv,  1909. 
A.  G.  Pohlman:  "The  Course  of  the  Blood  through  the  Fetal  Mammalian  Heart," 

Anat.  Record,  n,  1908. 
F.  Reagan:  "The  Fifth  Aortic  Arch  of  Mammalian  Embryos."  Amer.  Journ.  Anat... 

xii,  1912. 

E.  Retterer:  "Sur  la  part  que  prend  1' epithelium  a  la  formation  de  la  bourse  de 

Fabricius,  des  amygdales  et  des  plaques  de  Peyer,"  Journ.  de  I' Anat.  et  de  la 

Physiol.,  xxix,  1893. 
R.  Retzer:  "Some  Results  of  Recent  Investigations  on  the  Mammalian  Heart," 

Anat.  Record,  11,  1908. 
C.  Rose:  "Zur  Entwicklungsgeschichte  des  Saugethierherzens,"  Morphol.  Jahrbuch, 

xv,  1889. 
Florence  R.  Sabln:  "On  the  Origin  of  the  Lymphatic  System  from  the  Veins  and 

the  Development  of  the  Lymph  Hearts  and  Thoracic  Duct  in  the  Pig,"  Amer. 

Journ.  of  Anat.,  I,  1902. 
Florence  R.  Sabin:  "The  Development  of  the  Lymphatic  Nodes  in  the  Pig  and 

their  Relation  to  the  Lymph  Hearts,"  Amer.  Journ.  Anat.,  rv,  1905. 
Florence  R.  Sabin:  "Further  Evidence  on  the  Origin  of  the  Lymphatic  Endothelium 

from  the  Endothelium  of  the  Blood  Vascular  System,"  Anat.  Record,  11,  1908. 
Florence   R.  Sabin:    On  the  Development  of   the  Lymphatic   System   in  Human 
>  Embryos  with  a  Consideration  of  the  Morphology  of  the  System  as  a  Whole," 

Amer.  Journ.  Anat.,  ix,  1909. 
Florence  R.  Sabin:  "A  Critical  Study  of  the  Evidence  Presented  in  Several  Recent 

Articles  on  the  Development  of  the  Lymphatic  System,"  Anat.  Record,  v,  1911. 

F.  Saxer:  "Ueber  die  Entwicklung  und  der  Bau  normaler  Lymphdrusen  und  die 

Entsehung  der  roten  und  weissen  Blutkorperchen,"  Anat.  Hefte,  vi,  1896. 
H.  Schridde:  "Die  Entstehung  der  ersten  embryonalen  Blutzellen  des  Menschen," 

Folia  hcematol,  rv,  1907. 
P.  Stohr:  "Ueber  die  Entwicklung  der  Darmlymphknotchen  und  iiber  die  Riick- 

bildung  von  Darmdrusen,"  Archiv  fur  mikrosk.  Anat.,  LI,  1898. 
O.  van  der  Stricht:  "  Nouvelles  recherches  sur  la  genese  des  globules  rouges  et  des 

globules  blancs  du  sang,"  Archives  de  Biolog.,  xn,  1892. 
O.  van  der  Stricht:  "De  la  premiere  origine  du  sang  et  des  capillaires  sanguins  dans 

l'aire  vasculaire  du  Lapin,"  Comptes  Rendus  de  la  Soc.  de  Biolog.  Paris,  -Ser.  10, 

11,  1895. 
O.  Stoerk:  "Ueber  die  Chromreaktion  der  Glandula  coccygea  und  die  Beziehung, 

dieser  Druse  zum  Nervus  sympathicus,"  Arch,  fur  mikroskop.  Anat.,  lxix,  1906. 
J.  Tandler:  "Zur  Entwicklungsgeschichte  der  Kopfarterien  bei  den  Mammalia." 

Morphol.  Jahrbuch,  xxx,  1902. 


LITERATURE  279 

J.  Tandler:  "Zur  Entwickelungsgeschichte  der  menschlichen  Darmarterien,"   Anat. 

Hefte,  xxiii,  1903. 
J.  Tandler:  "  Ueber  die  Varietaten  der  arteria  coeliaca  und  deren  Entwicklung,"  Anat. 

Hefte,  xxv,  1904. 
J.  Tandler:   "  Ueber  die  Entwicklung   des  fiinften  Aortenbogens  und  der  fiinften 

Schlundtasche  beim  Menschen,"  Anat.  Hefte,  xxxvin,  1909. 
W.  Tonkoff:  "  Die  Entwickelung  der  Milz  bei  den  Amnioten,"  Arch,  fiir  mikrosk. 

Anat.,  lvi,  1900. 
Bertha  de  Vriese:  "Recherches  sur  revolution  des  vaissaux  sanguins  des  membres 

chez  l'homme,"  Archives  de  Biolog.,  xvili,  1902. 
F.  Weidenreich:  "Die  roten  Blutkorperchen,"  Ergeb.  Anat.  und  Entwick.,  xiii,  1903 

xiv,  1904. 
F.  Weidenreich:  "Die  Leucocyten  und  verwandte  zellformen,"  Ergeb.  Anat.  und; 

Entwick.,  xvi,  191 1. 
J.  H.  Wright:  "The  Histogenesis  of  the  Blood  Platelets,"  Journ.  of  Morph.,  xxr,  1910. 


CHAPTER  X. 

THE  DEVELOPMENT  OF  THE  DIGESTIVE 
TRACT  AND  GLANDS. 

The  greatest  portion  of  the  digestive  tract  is  formed  by  the  con- 
striction off  of  the  dorsal  portion  of  the  yolk-sac,  as  shown  in  Fig.  52, 
the  result  being  the  formation  of  a  cylinder,  closed  at  either  end, 
and  composed  of  a  layer  of  splanchnic  mesoderm  lined  on  its  inner 
surface  by  endoderm.  This  cylinder  is  termed  archenteron  and  has 
connected  with  it  the  yolk-stalk  and  the  allantois,  the  latter  com- 
municating with  its  somewhat  dilated  terminal  portion,  which  also 
receives  the  ducts  of  the  primitive  kidneys  and  is  known  as  the 
cloaca  (Fig.  170). 

At  a  very  early  stage  of  development  the  anterior  end  of  the 
embryo  begins  to  project  slightly  in  front  of  the  yolk-sac,  so  that  a 
shallow  depression  is  formed  between  the  two  structures.  As  the 
constriction  of  the  embryo  from  the  sac  proceeds,  the  anterior  portion 
of  the  brain  becomes  bent  ventrally  and  the  heart  makes  its  appear- 
ance immediately  in  front  of  the  anterior  surface  of  the  yolk-sac, 
and  so  the  depression  mentioned  above  becomes  deepened  (Fig.  169) 
to  form  the  oral  sinus.  The  floor  of  this,  lined  by  ectoderm,  is 
immediately  opposite  the  anterior  end  of  the  archenteron,  and,  since 
mesoderm  does  not  develop  in  this  region,  the  ectoderm  of  the  sinus 
and  the  endoderm  of  the  archenteron  are  directly  in  contact,  forming 
a  thin  pharyngeal  membrane  separating  the  two  cavities  (Fig.  169,  pm) 
In  embryos  of  2.15  mm.  this  membrane  is  still  existent,  but  soon  after 
it  becomes  perforated  and  finally  disappears,  so  that  the  archenteron 
and  oral  sinus  become  continuous. 

Toward  its  posterior  end  trr;  archenteron  comes  into  somewhat 
similar  relations  with  the  ectoderm,  though  a  marked  difference  is 
noticeable  in  that  the  area  over  which  the  cloacal  endoderm  is  in 

280 


DEVELOPMENT    OF   THE    DIGESTIVE    TRACT 


281 


O-A 


contact  with  the  ectoderm  to  form  the  cloacal  membrane  (Fig.  170,  cm) 
lies  a  little  in  front  of  the  actual  end  of  the  archenteric  cylinder,  the 
portion  of  the  latter  which  lies  posterior  to  the  membrane  forming 
what  has  been  termed  the  postanal  gut  {p. an).  This  diminishes  in 
size  during  development  and  early  disappears  altogether,  and  the 
pouch-like  fold  seen  in  Fig.  170  between  the  intestinal  portion  of  the 
archenteron  and  the  allantoic  stalk  (al)  deepening  until  its  floor 
comes  into  contact  with  the  cloa- 
cal membrane,  the  cloaca  be- 
comes divided  into  a  ventral  por- 
tion, with  which  the  allantois 
and  the  primitive  excretory  ducts 
(w)  are  connected,  and  a  dorsal 
portion  which  becomes  the  lower 
end  of  the  rectum.  This  latter 
abuts  upon  the  dorsal  portion 
of  the  cloacal  membrane,  and 
this  eventually  ruptures,  so  that 
the  posterior  communication  of 
the  archenteron  with  the  exterior 
becomes  established.  This  rup- 
ture, however,  does  not  occur  un- 
til a  comparatively  late  period  of 
development,  until  after  the  em- 
bryo has  reached  the  fetal  stage; 
nor  does  the  position  of  the  membrane  correspond  with  the  adult 
anus,  since  later  there  is  a  considerable  development  of  mesoderm 
around  the  mouth  of  the  cloaca,  bulging  out,  as  it  were,  the  sur- 
rounding ectoderm,  more  especially  anteriorly  where  it  forms  the 
large  genital  tubercle  (see  Chapter  XIII),  and  posteriorly  where  it  pro- 
duces the  anal  tubercle.  This  appears  as  a  rounded  elevation  on 
each  side  of  the  median  line,  immediately  behind  the  cloacal  mem- 
brane and  separated  from  the  root  of  the  caudal  projection  by  a  de- 
pression, the  precaudal  recess.  Later  the  two  elevations  unite  across 
the  median  line  to  form  a  transverse  ridge,  the  ends  of  which  curve 


Fig.  169. — Reconstruction  of  the 
Anterior  Portion  of  an  Embryo  of  2.15 

MM. 

ab.  Aortic  bulb;  h,  heart;  0,  auditory  cap- 
sule; op,  optic  evagination;/>?w,  pharyngeal 
membrane. — {His.) 


282 


DIGESTIVE    TRACT  AND   GLANDS 


forward  and  eventually  meet  in  front  of  the  original  anal  orifice. 
From  the  mesoderm  of  the  circular  elevation  thus  produced  the  ex- 
ternal sphincter  ani  muscle  is  formed,  and  it  would  seem  that  so 
much  of  the  lower  end  of  the  rectum  as  corresponds  to  this  muscle 
is  formed  by  the  inner  surface  of  the  elevation  and  is  therefore 
ectodermal.  The  definitive  anus  being  at  the  end  of  this  terminal 
portion  of  the  gut  is  therefore  some  distance  away  from  the  posi- 
tion of  the  original  cloacal  membrane. 


nc 


Fig.  170. — Reconstruction  of  the  Hind  End  of  an  Embryo  6.5  mm.  Long 

al,  Allantois;  b,  belly-stalk;  cl,  cloaca;  cm,  cloacal  membrane;  i,  intestine;  n,  spinal 
cord;  nc,  notochord;  p.an,  postanal  gut;  ur,  outgrowth  to  form  ureter  and  metanephros; 
w,  Wolffian  duct. — (Keibel.) 


It  will  be  noticed  that  the  digestive  tract  thus  formed  consists  of 
three  distinct  portions,  an  anterior,  short,  ectodermal  portion,  an 
endodermal  portion  representing  the  original  archenteron,  and  a 
posterior  short  portion  which  is  also  ectodermal.  The  differentia- 
tion of  the  tract  into  its  various  regions  and  the  formation  of  the 
various  organs  found  in  relation  with  these  may  now  be  considered. 


DEVELOPMENT   OF   THE    MOUTH   REGIONS  283 

The  Development  of  the  Mouth  Region. — The  deepening 
of  the  oral  sinus  by  the  development  of  the  first  branchial  arch  and 
its  separation  into  the  oral  and  nasal  cavities  by  the  development 
of  the  palate  have  already  been  described  (p.  99),  but,  for  the  sake 
of  continuity  in  description,  the  latter  process  may  be  briefly  recalled. 
At  first  the  nasal  pits  communicate  with  the  oral  sinus  by  grooves 
lying  one  on  each  side  of  the  fronto-nasal  process,  but  by  the  union 
of  the  latter,  through  its  processus  globularis,  with  the  maxillary 
processes  these  communications  are  interrupted  and  the  floors  of 
the  nasal  pits  are  separated  from  the  oral  cavity  by  thin  bucco-nasal 
membranes,  formed  of  the  nasal  epithelium  in  contact  with  that 
of  the  oral  cavity.  In  embryos  of  about  15  mm.  these  membranes 
break  through  and  disappear,  and  the  nasal  and  oral  cavities  are 
again  in  communication,  but  the  communications  are  now  behind 
the  maxillary  processes  and  constitute  what  are  termed  the  primitive 
choance.  The  oral  cavity  at  this  stage  does  not,  however,  correspond 
with  the  adult  mouth  cavity,  since  there  is  as  yet  no  palate,  the  roof 
of  the  oral  cavity  being  the  base  of  the  skull.  From  the  maxillo- 
palatine  portions  of  the  upper  jaw,  shelf-like  ridges  begin  to  grow, 
being  at  first  directed  downward  so  that  their  surfaces  are  parallel 
with  the  sides  of  the  tongue,  which  projects  up  between  them. 
Later,  however,  they  become  bent  upward  to  a  horizontal  position 
(Fig.  171)  and  eventually  meet  in  the  median  line  to  form  the  palate, 
separating  the  nasal  cavities  from  the  mouth  cavity.  All  that  por- 
tion of  the  original  oral  cavity  which  lies  behind  the  posterior  edge 
of  the  palatal  shelf  is  now  known  as  the  pharynx,  the  boundary 
between  this  and  the  mouth  cavity  being  emphasized  by  the  pro- 
longation backward  and  downward  of  the  posterior  angles  of  the 
palatal  shelf  as  ridges,  which  form  the  pharyn  go -palatine  arches 
{posterior  pillars  of  the  fauces) .  The  nasal  cavities  now  communicate 
with  the  upper  part  of  the  pharynx  (naso-pharynx)  by  the  posterior 
choanae.  The  palatal  processes  are  entirely  derived  from  the 
maxillary  processes,  the  premaxillary  portion  of  the  upper  jaw, 
which  is  a  derivative  of  the  fronto-nasal  processes,  not  taking  part 
in  their  formation/    Consequently  a  gap  exists  between  the  palatal 


284 


DEVELOPMENT    OE    THE    MOUTH    REGIONS 


shelves  and  the  premaxillae  for  a  time,  by  which  the  nasal  and 
mouth  cavities  communicate;  it  places  the  organ  of  Jacobson  (see 
p.  429)  in  communication  with  the  mouth  cavity  and  may  persist 
until  after  birth.  Later  it  becomes  closed  over  by  mucous  mem- 
brane, but  may  be  recognized  in  the  dried  skull  as  the  foramen 
incisivum  (anterior  palatine  canal). 

Occasionally  there  is  a  failure  of  the  union  of  the  palatal  plates,  the 
condition  known  as  cleft  palate  resulting.  The  inhibition  of  development 
which  brings  about  this  condition  may  take  place  at  different  stages,  but 
frequently  it  occurs  while  the  plates  still  have  an  almost  vertical  direction. 
Typically  cleft  palate  is  a  deficiency  in  the  median  line  of  the  roof  of  the 


Fig.  171. — View  of  the  Roof  of  the  Oral  Fossa  of  Embryo  showing  the  Lip- 
groove  and  the  Formation  of  the  Palate. — (His.) 

mouth,  not  affecting  the  upper  jaw,  but  very  frequently  it  is  combined 
with  the  defect  which  produces  hare-lip  (see  p.  100),  in  which  case  the 
cleft  may  be  continued  through  the  upper  jaw  between  its  maxillary  and 
premaxillary  portions  on  either  or  both  sides,  according  to  the  extent  of 
the  defect. 

At  about  the  fifth  week  of  development  a  downgrowth  of  epi- 
thelium into  the  substance  of  both  the  maxillary  and  fronto-nasal 
processes  above  and  the  mandibular  process  below  takes  place,  and 
the  surface  of  the  downgrowth  becomes  marked  by  a  deepening 
groove  (Fig.  171),  which  separates  an  anterior  fold,  the  Up,  from 
the  jaw  proper  (Fig.  172).     Mention  should  also  be  made  of  the 


DEVELOPMENT    OF   THE    TEETH  2S5 

fact  that  at  an  early  stage  of  development  a  pouch  is  formed  in  the 
median  line  of  the  roof  of  the  oral  sinus,  just  in  front  of  the  pharyn- 
geal membrane,  by  an  outgrowth  of  the  epithelium.  This  pouch, 
known  as  Rathke's  pouch,  comes  in  contact  above  with  a  downgrowth 
from  the  floor  of  the  brain  and  forms  with  it  the  pituitary  body 
(seep.  399). 

The  Development  of  the  Teeth. — When  the  epithelial  downgrowth 
which  gives  rise  to  the  lip  groove  is  formed,  a  horizontal  outgrowth 
develops  from  it  which  extends  backward  into  the  substance  of  the 
jaw,  forming  what  is  termed  the  dental  shelf  (Fig.  172,  A).  This 
at  first  is  situated  on  the  anterior  surface  of  the  jaw,  but  with  the 
continued  development  of  the  lip  fold  it  is  gradually  shifted  until  it 
comes  to  lie  upon  the  free  surface  (Fig.  172,  B),  where  its  superficial 
edge  is  marked  by  a  distinct  groove,  the  dental  groove  (Fig.  171). 
At  first  the  dental  shelf  of  each  jaw  is  a  continuous  plate  of  cells, 
uniform  in  thickness  throughout  its  entire  width,  but  later  ten  thick- 
enings develop  upon  its  deep  edge,  and  beneath  each  of  these  the 
mesoderm  condenses  to  form  a  dental  papilla,  over  the  surface  of 
which  the  thickening  moulds  itself  to  form  a  cap,  termed  the  enamel 
organ  (Fig.  172,  B).  These  ten  papillae  in  each  jaw,  with  their 
enamel  caps,  represent  the  teeth  of  the  first  dentition. 

The  papillae  do  not,  however,  project  into  the  very  edge  of  the 
dental  shelf,  but  obliquely  into  what,  in  the  lower  jaw,  was  originally 
its  under  surface  (Fig.  172,  B),  so  that  the  edge  of  the  shelf  is  free 
to  grow  still  deeper  into  the  surface  of  the  jaw.  This  it  does,  and 
upon  the  extension  so  formed  there  is  developed  in  each  jaw  a  second 
set  of  thickenings,  beneath  each  of  which  a  dental  papilla  again 
appears.  These  tooth-germs  represent  the  incisors,  canines,  and 
premolars  of  the  permanent  dentition.  The  lateral  edges  of  the 
dental  shelf  being  continued  outward  toward  the  articulations  of 
the  jaws  as  prolongations  which  are  not  connected  with  the  surface 
epithelium,  opportunity  is  afforded  for  the  development  of  three 
additional  thickenings  on  each  side  in  each  jaw,  and,  papillae  devel- 
oping beneath  these,  twelve  additional  tooth-germs  are  formed. 
These  represent  the  permanent  molars;  their  formation  is  much 


286 


DEVELOPMENT    OF   THE    TEETH 


later  than  that  of  the  other  teeth,  the  germ  of  the  second  molar  not 
appearing  until  about  the  sixth  week  after  birth,  while  that  of  the 
third  is  delayed  until  about  the  fifth  year. 

As  the  tooth-germs  increase  in  size,  they  approach  nearer  and 
nearer  to  the  surface  of  the  jaw,  and  at  the  same  time  the  enamel 
organs  separate  from  the  dental  shelf  until  their  connection  with  it 
is  a  mere  neck  of  epithelial  cells.  In  the  meantime  the  dental  shelf 
itself  has  been  undergoing  degeneration  and  is  reduced  to  a  reticulum 


W'-:^^^^^0^:i'- 


Mill 


3 


Fig.  172. — Transverse  Sections  through  the  Lower  Jaw  showing  the 
Formation  of  the  Dental  Shelf  in  Embryos  of  (A)  17  mm.  and  (B)  40  mm. — 
(Rose.) 

which  eventually  completely  disappears,  though  fragments  of  it  may 
occasionally  persist  and  give  rise  to  various  malformations.  With 
the  disappearance  of  the  last  remains  of  the  shelf,  the  various  tooth- 
germs  naturally  lose  all  connection  with  one  another. 

It  will  be  seen,  from  what  has  been  said,  that  each  tooth-germ 
consists  of  two  portions,  one  of  which,  the  enamel  organ,  is  derived 
from  the  ectoderm,  while  the  other,  the  dental  papilla,  is  mesen- 


DEVELOPMENT    OF   THE    TEETH  287 

chymatous.  Each  of  these  gives  rise  to  a  definite  portion  of  the 
fully  formed  tooth,  the  enamel  organ,  as  its  name  indicates,  produc- 
ing the  enamel,  while  from  the  dental  papilla  the  dentine  and  pulp 
are  formed. 

The  cells  of  the  enamel  organ  which  are  in  contact  with  the  sur- 
face of  the  papilla,  at  an  early  stage  assume  a  cylindrical  form  and 
become  arranged  in  a  definite  layer,  the  enamel  membrane  (Fig. 
173,  SEi),  while  the  remaining  cells  (SEa)  apparently  degenerate 
eventually,  though  they  persist  for  a  time  to  form  what  has  been 
termed  the  enamel  pulp.  The  formation  of  the  enamel  seems  to  be 
due  to  the  direct  transformation  of  the  enamel  cells,  the  process  begin- 
ning at  the  basal  portion  of  each  cell,  and  as  a  result,  the  enamel 
consists  of  a  series  of  prisms,  each  of  which  represents  one  of  the 
cells  of  the  enamel  membrane.  The  transformation  proceeds 
until  the  cells  have  become  completely  converted  into  enamel 
prisms,  except  at  their  very  tips,  which  form  a  thin  membrane,  the 
enamel  cuticle,  which  is  shed  soon  after  the  eruption  of  the  teeth. 

The  dental  papillae  are  at  first  composed  of  a  closely  packed  mass 
of  mesenchyme  cells,  which  later  become  differentiated  into  connec- 
tive tissue  into  which  blood-vessels  and  nerves  penetrate.  The 
superficial  cells  form  a  more  or  less  definite  layer  (Fig.  173,  od), 
and  are  termed  odontoblasts,  having  the  function  of  manufacturing 
the  dentine.  This  they  accomplish  in  the  same  manner  as  that  in 
which  the  periosteal  osteoblasts  produce  bone,  depositing  the  den- 
tine between  their  surfaces  and  the  adjacent  surface  of  the  enamel. 
The  outer  surface  of  each  odontoblast  is  drawn  out  into  a  number 
of  exceedingly  fine  processes  which  extend  into  the  dentine  to  occupy 
the  minute  dentinal  tubules,  just  as  processes  of  the  osteoblasts 
occupy  the  canaliculi  of  bone. 

At  an  early  stage  the  enamel  membrane  forms  an  almost  com- 
plete investment  for  the  dental  papilla  (Fig.  173),  but  as  the  ossifi- 
cation of  the  tooth  proceeds,  it  recedes  from  the  lower  part,  until 
finally  it  is  confined  entirely  to  the  crown.  The  dentine  forming  the 
roots  of  the  tooth  then  becomes  enclosed  in  a  layer  of  cement,  which 
is  true  bone  and  serves  to  unite  the  tooth  firmly  to  the  walls  of  its 


288 


DEVELOPMENT    OF    THE    TEETH 


socket.  As  the  tooth  increases  in  size,  its  extremity  is  brought 
nearer  to  the  surface  of  the  gum  and  eventually  breaks  through,  the 
eruption  of  the  first  teeth  usually  taking  place  during  the  last  half 
of  the  first  year  after  birth.     The  growth  of  the  permanent  teeth 


-£p. 


-Od. 


Fig.  173. — Section  through  the  First  Molar  Tooth  of  a  Rat,  Twelve  Days  Old. 
Ap,  Periosteum;  E,  dentine;  Ep,  epidermis;  Od,  odontoblasts;  S,  enamel;  SEa 
and  SEi,  outer  and  inner  layers  of  the  enamel  organ;  SE,  portion  of  the  enamel  organ 
which  does  not  produce  enamel. — (von  Brunn.) 


proceeds  slowly  at  first,  but  later  it  becomes  more  rapid  and  pro- 
duces pressure  upon  the  roots  of  the  primary  teeth.  These  roots 
then  undergo  partial  absorption,  and  the  teeth  are  thus  loosened 


DEVELOPMENT  OF  THE  TONGUE  289 

in  their  sockets  and  are  readily-  pushed  out  by  the  further  growth  of 
the  permanent  teeth. 

The  dates  and  order  of  the  eruption  of  the  teeth  are  subject  to  con- 
siderable variation,  but  the  usual  sequence  is  somewhat  as  follows: 

Primary  Dentition. 

Median  incisors 6th  to  8th  month. 

Lateral  incisors 8th  to  12  month. 

First  molars Beginning  of  2d  year. 

Canines i£  years. 

Second  molars 3  to  3^  years. 

The  teeth  of  the  lower  jaw  generally  precede  those  of  the  upper. 

Permanent  Dentition. 

First  molars 7th  year. 

Middle  incisors 8th  year. 

Lateral  incisors 9th  year. 

First  premolars 10th  year. 

Second  premolars nth  year. 

Canines 


13th  to  14th  years. 

Second  molars  J 

Third  molars 17th  to  40th  years. 

In  a  considerable  percentage  of  individuals  the  third  molars  (wisdom 
teeth)  never  break  through  the  gums,  and  frequently  when  they  do  so 
they  fail  to  reach  the  level  of  the  other  teeth,  and  so  are  only  partly  func- 
tional. These  and  other  peculiarities  of  a  structural  nature  shown 
by  these  teeth  indicate  that  they  are  undergoing  a  retrogressive  evolution. 

The  Development  of  the  Tongue. — Strictly  speaking,  the 
tongue  is  largely  a  development  of  the  pharyngeal  region  of  the 
digestive  tract  and  only  secondarily  grows  forward  into  the  floor  of 
the  mouth.  In  embryos  of  about  3  mm.  there  may  be  seen  in  the 
median  line  of  the  floor  of  the  mouth,  between  the  ventral  ends  of 
the  first  and  second  branchial  arches,  a  small  rounded  elevation 
which  has  been  termed  the  tuberculum  impar  (Fig.  174,  Ti).  It  was 
at  one  time  believed  that  this  gave  rise  to  the  anterior  portion  of  the 
tongue,  but  recent  observations  seem  to  show  that  it  reaches  its 
greatest  development  in  embryos  of  about  8  mm.,  after  which  it 
becomes  less  prominent  and  finally  unrecognizable.  But  before 
19 


290 


DEVELOPMENT   OF   THE    TONGUE 


this  occurs  a  swelling  appears  in  the  anterior  part  of  the  mouth  on 
each  side  of  the  median  line  (Fig.  174,  t),  and  these  gradually  increase 


n 


X-' 


x 


-Cap 


/ 


/ 


/<■ 


Fig.  174. — Floor  of  the  Mouth  and  Pharynx  of  an  Embryo  of  7.5  mm.,  from 

a  Reconstruction. 

Cop,  Copula;  /,  furcula;  t,  swelling  that  gives  rise  to  the  body  of  the  tongue;  Ti, 

tuberculum  impar;  I-III,  branchial  arches. 

in  size  and  eventually  unite  in  the  median  line  to  form  the  main 
mass  of  the  body  of  the  tongue.     They  are  separated  from  the 

neighboring  portions  of  the  first 
branchial  arch  by  a  deep  groove, 
the  alveolo-lingual  groove,  and  pos- 
teriorly are  separated  from  the 
second  arch  by  a  groove  which  la- 
ter becomes  distinctly  V-shaped 
(Fig.  175),  a  deep  depression,  which 
gives  rise  to  the  thyreoid  body  lying 
at  the  apex  of  the  V.  Behind  the 
thyreoid  pouch  the  ventral  ends  of 
the  second  and  third  branchial  arches 
unite  to  form  an  elevation,  the 
copula  (Fig.  174,  cop),  and  from  this 
and  the  adjacent  portions  of  the 
second  and  third  arches  the  posterior  portion  of  the  tongue  develops. 
The  tongue  then  consists  of  two  distinct  portions,  which  even- 


Fig.  175. — The  Floor  of  the 
Pharynx  of  an  Embryo  of  about 
20  MM. 

ep,  Epiglottis;  fc,  foramen  caecum; 
t1  and  t2  median  and  lateral  portions 
of  the  tongue. — (His.) 


THE    SALIVARY    GLANDS  2  9 1 

tually  fuse  together,  but  the  groove  which  originally  separated  them 
remains  more  or  less  clearly  distinguishable  (Fig.  175),  the  vallate 
papillae  (see  p.  430)  developing  immediately  anterior  to  it. 

The  tongue  is  essentially  a  muscular  organ,  being  formed  of  a  central 
mass  of  muscular  tissue,  enclosed  at  the  sides  and  dorsally  by  mucous 
membrane  derived  from  the  floor  of  the  mouth  and  pharynx.  The 
muscular  tissue  consists  partly  of  fibers  limited  to  the  substance  of  the 
tongue  and  forming  the  m.  lingualis,  and  also  of  a  number  of  extrinsic 
muscles,  the  hyoglossi,  genioglossi,  styloglossi,  glos  so  palatini,  and  chondro- 
glossi.  The  last  two  muscles  are  innervated  by  the  vagus  nerve,  and 
the  remaining  extrinsic  muscles  receive  fibers  from  the  hypoglossal,  while 
the  lingualis  is  supplied  partly  by  the  hypoglossal  and  partly,  apparently, 
by  the  facial  through  the  chorda  tympani.  That  the  facial  should  take 
part  in  the  supply  is  what  might  be  expected  from  the  mode  of  develop- 
ment of  the  tongue,  but  the  hypoglossal  has  been  seen  to  correspond  to 
certain  primarily  postcranial  metameres  (p.  169),  and  its  relation  to 
structures  taking  part  in  the  formation  of  an  organ  belonging  to  the  anterior 
part  of  the  pharynx  seems  somewhat  anomalous.  It  may  be  supposed 
that  in  the  evolution  of  the  tongue  the  extrinsic  muscles,  together  with  a 
certain  amount  of  the  lingualis,  have  grown  into  the  tongue  thickenings 
from  regions  situated  much  further  back,  for  the  most  part  from  behind 
the  last  branchial  arch. 

Such  an  invasion  of  the  tongue  by  muscles  from  posterior  segments 
would  explain  the  distribution  of  its  sensory  nerves  (Fig.  176).  The 
anterior  portion,  from  its  position,  would  naturally  be  supplied  by  branches 
from  the  fifth  and  seventh  nerves,  while  the  posterior  portion  might  be 
expected  to  be  supplied  by  the  seventh.  There  seems,  however,  to  have 
been  a  dislocation  forward,  if  it  may  be  so  expressed,  of  the  mucous  mem- 
brane, the  sensory  distribution  of  the  ninth  nerve  extending  forward  upon 
the  posterior  part  of  the  anterior  portion  of  the  tongue,  while  a  consider- 
able amount  of  the  posterior  portion  is  supplied  by  the  tenth  nerve. 
The  distribution  of  the  sensory  fibers  of  the  facial  is  probably  confined 
entirely  to  the  anterior  portion,  though  further  information  is  needed  to 
determine  the  exact  distribution  of  both  the  motor  and  sensory  fibers  of 
this  nerve  in  the  tongue. 

The  Development  of  the  Salivary  Glands. — In  embryos  of 
about  8  mm.  a  slight  furrow  may  be  observed  in  the  floor  of  the 
groove  which  connects  the  lip  grooves  of  the  upper  and  lower  jaws 
at  the  angle  of  the  mouth  and  may  be  known  as  the  cheek  groove. 
In  later  stages  this  furrow  deepens  and  eventually  becomes  closed 
in  to  form  a  hollow  tubular  structure,  which  in  embryos  of  17  mm. 


292 


THE    SALIVARY   GLANDS 


has  separated  from  the  epithelium  of  the  floor  of  the  cheek  groove 
except  at  its  anterior  end  and  has  become  embedded  in  the  connec- 
tive tissue  of  the  cheek.  This  tube  is  readily  recognizable  as  the 
parotid  gland  and  duct,  and  from  the  latter  as  it  passes  across  the 
masseter  muscle  a  pouch-like  outgrowth  is  early  formed  which  prob- 
ably represents  the  soda  parotidis. 


Fig.  176. — Diagram  of  the  Distribution  of  the  Sensory  Nerves  of  the  Tongue. 
The  area  supplied  by  the  fifth  (and  seventh)  nerve  is  indicated  by  the  transverse 
lines;  that  of  the  ninth  by  the  oblique  lines;  and  that  of  the  tenth  by  the  small  circles. 
— {Zander.) 

The  submaxillary  gland  and  duct  appear  in  embryos  of  about 
13  mm.  as  a  longitudinal  ridge-like  thickening  of  the  epithelium 
of  the  floor  of  the  alveolo-lingual  groove  (see  p.  290).     This  ridge 


THE    SALIVARY    GLANDS 


293 


gradually  separates  from  behind  forward  from  the  floor  of  the 
groove  and  sinks  into  the  subjacent  connective  tissue,  retaining, 
however,  its  connection  with  the  epithelium  at  its  anterior  end, 
which  indicates  the  position  of  the  opening  of  the  duct.  In  the 
vicinity  of  this  there  appear  in  embryos  of  24.4  mm.  five  small 
bud-like  downgrowths  of  the  epithelium  (Fig.  177,  SL),  which  later 
increase  considerably  in  number  as  well  as  in  size,  and  constitute  a 
group  of  glands  which  are  generally  spoken  of  as  the  sublingual 
gland. 

As  these  representatives  of  the  various  glands  increase  in  length, 


ZAl\ 


Man. 


Fig.  177. — Transverse  Section  of  the  Lower  Jaw  and  Tongue  of  an  Embryo 

of  about  20  mm. 
D,  Digastric  muscle;    GGl.,  genioglossus,  GH.\  geniohyoid;  T.Al,  inferior  alveolar 
nerve;  Man,  mandible;  MK,  Meckel's  cartilage;  My,  mylohyoid;  SL,  sublingual  gland; 
S.Mx,  submaxillary  duct;  T,  tongue. 

they  become  lobed  at  their  deeper  ends,  and  the  lobes  later  give 
rise  to  secondary  outgrowths  which  branch  repeatedly,  the  terminal 
branches  becoming  the  alveoli  of  the  glands.  A  lumen  early  ap- 
pears in  the  duct  portions  of  the  structures,  the  alveoli  remaining 
solid  for  a  longer  time,  although  they  eventually  also  become  hollow. 

It  is  to  be  noted  that  each  parotid  and  submaxillary  consists  of  a  single 
primary  outgrowth,  and  is  therefore  a  single  structure  and  not  a  union  of 
a  number  of  originally  separate  parts.     The  sublingual  glands  of  adult 


294  THE    PHARYNX 

anatomy  are  usually  described  as  opening  upon  the  floor  of  the  mouth  by 
a  number  of  separate  ducts.  This  arises  from  the  fact  that  the  majority 
of  the  glands  which  form  in  the  vicinity  of  the  opening  of  Wharton's 
duct  remain  quite  small,  only  one  of  them  on  each  side  giving  rise  to  the 
sublingual  gland  proper.  The  small  glands  have  been  termed  the 
alveolo-lingual  glands,  and  each  one  of  them  is  equivalent  to  a  parotid  or 
submaxillary  gland.  In  other  words,  there  are  in  reality  not  three  pairs 
of  salivary  glands,  but  from  fourteen  to  sixteen  pairs,  there  being  usually 
from  eleven  to  thirteen  alveolo-lingual  glands  on  each  side. 

The  Development  of  the  Pharynx. — The  pharynx  represents 
the  most  anterior  part  of  the  archenteron,  that  portion  in  which  the 
branchial  arches  develop,  and  in  the  embryo  it  is  relatively  much 
longer  than  in  the  adult,  the  diminution  being  brought  about  by 
the  folding  in  of  the  posterior  arches  and  the  formation  of  the  sinus 
prsecervicalis  already  described  (p.  97).  Between  the  various 
branchial  arches,  grooves  occur,  representing  the  endodermal 
portions  of  the  grooves  which  separate  the  arches.  During  develop- 
ment the  first  of  these  becomes  converted  into  the  tympanic  cavity 
of  the  ear  and  the  Eustachian  tube  (see  Chapter  XV) ;  the  second 
disappears  in  its  upper  part,  the  lower  persisting  as  the  fossa  in 
which  the  tonsil  is  situated;  while  the  lower  parts  of  the  remaining 
two  are  represented  by  the  sinus  piriformis  of  the  larynx  (His),  and 
also  leave  traces  of  their  existence  in  detached  portions  of  their  epi- 
thelium which  form  what  are  termed  the  branchial  epithelial  bodies , 
and  take  part  in  the  formation  of  the  thyreoid  and  thymus  glands. 

In  the  floor  of  the  pharynx  behind  the  thickenings  which  pro- 
duce the  tongue  there  is  to  be  found  in  early  stages  a  pair  of  thick- 
enings passing  horizontally  backward  and  uniting  in  front  so  that 
they  resemble  an  inverted  U  (Fig.  178,  /).  These  ridges,  which 
form  what  is  termed  the  furcula  (His),  are  concerned  in  the  forma- 
tion of  parts  of  the  larynx  (see  p.  334).  In  the  part  of  the  roof  of 
the  pharynx  which  comes  to  lie  between  the  openings  of  the  Eusta- 
chian tubes,  a  collection  cf  lymphatic  tissue  takes  place  beneath 
the  mucous  membrane,  forming  the  pharyngeal  tonsil,  and  imme- 
diately behind  this  there  is  formed  in  the  median  line  an  upwardly 
projecting  pouch,  the  pharyngeal  bursa,  first  certainly  noticeable 
in  embryos  6.5  mm.  in  length. 


THE    BRANCHIAL   EPITHELIAL    BODIES 


295 


This  bursa  has  very  generally  been  regarded  as  the  persistent  remains 
of  Rathke's  pouch  (p.  285),  especially  since  it  is  much  more  pronounced 
in  fetal  than  in  adult  life.  It  has  been  shown,  however,  that  it  is  formed 
quite  independently  of  and  posterior  to  the  true  Rathke's  pouch  (Killian), 
though  what  its  significance  may  be  is  still  uncertain. 

The  tonsils  are  formed  from  the  epithelium  of  the  second  bran- 
chial groove.  At  about  the  fourth  month  solid  buds  begin  to  grow 
from  the  epithelium  into  the  subjacent  mesenchyme,  and  depressions 
appear  on  the  surface  of  this  region.  Later  the  buds  become  hollow 
by  a  cornification  of  their  central  cells,  and  open  upon  the  floor  of 
the  depressions  which  represent  the 
crypts  of  the  tonsil.  In  the  meantime 
lymphocytes,  concerning  whose  origin 
there  is  a  difference  of  opinion,  collect  in 
the  subjacent  mesenchyme  and  eventu- 
ally aggregate  to  form  lymphatic  follicles 
in  close  relation  with  the  buds.  Whether 
the  lymphocytes  wander  out  from  the 
blood  into  the  mesenchyme  or  are  derived 
directly  from  the  epithelium  or  the  mes- 
enchyme cells  is  the  question  at  issue. 

The  tonsil  may  grow  to  a  size  sufficient 
to  fill  up  completely  the  groove  in  which 
it  forms,  but  not  infrequently  a  marked 
depression,  the  fossa  supratonsillaris,  exists  above  it  and  represents 
a  portion  of  the  original  second  branchial  furrow. 

The  groove  of  Rosenmuller,  which  was  at  one  time  thought  to  be 
also  a  remnant  of  the  second  furrow,  is  a  secondary  depression 
which  appears  in  embryos  of  11.5  cm.  behind  the  opening  of  the 
Eustachian  tube,  in  about  the  region  of  the  third  branchial  furrow. 

The  Development  of  the  Branchial  Epithelial  Bodies. — These  are 
structures  which  arise  either  as  thickenings  or  as  outpouchings  of 
the  epithelium  lining  the  lower  portions  of  the  inner  branchial  fur- 
rows. Five  pairs  of  these  structures  are  developed  and,  in  addition, 
there  is  a  single  unpaired  median  body.  This  last  makes  its  appear- 
ance in  embryos  of  about  3  mm.,  and  gives  rise  to  the  major  por- 


Fig.  178. — The  Floor  of 
the  Pharynx  of  an  Embryo 
of  2.15  MM. 

/.  Furcula;  t,  tuberculum  im- 
pair.— (His.) 


296 


THE    BRANCHIAL   EPITHELIAL   BODIES 


tion  of  the  thyreoid  body.  It  is  situated  immediately  behind  the 
anterior  portion  of  the  tongue,  at  the  apex  of  the  groove  between 
this  and  the  posterior  portion,  and  is  first  a  slight  pouch -like  depres- 
sion. As  it  deepens,  its  extremity  becomes  bilobed,  and  after  the 
embryo  has  reached  a  length  of  6  mm.  it  becomes  completely  sepa- 
rated from  the  floor  of  the  pharynx.  The  point  of  its  original 
origin  is,  however,  permanently  marked  by  a  circular  depression, 
the  foramen  cacum  (Fig.  175,  fc).  Later  the  bilobed  body  migrates 
down  the  neck  and  becomes  a  solid  transversely  elongated  mass 
(Fig.  179,  th),  into  the  substance  of  which  trabecule  of  connective 
tissue  extend,  dividing  it  into  a  network  of  anastomosing  cords  which 


Fig.  179. — Reconstructions  of  the  Branchial  Epithelial  Bodies  of  Embryos. 

of  (a)  14  mm.  and  (b)  26  mm. 

ao,  Aorta;  Ith,  lateral  thyreoid;  ph,  pharynx;  pth1  and  pth2,  parathyreoids;  th,  thyreoid; 

thy,  thymus;  vc,  vena  cava  superior. — (Tourneux  and  Verdun.) 

later  divide  transversely  to  form  follicles.  When  the  embryo  has 
reached  a  length  of  2.6  cm.,  a  cylindrical  outgrowth  arises  from  the 
anterior  surface  of  the  mass,  usually  a  little  to  the  left  of  the  median 
line,  and  extends  up  the  neck  a  varying  distance,  forming,  when  it 
persists  until  adult  life,  the  so-called  pyramid  of  the  thyreoid  body. 

This  account  of  the  pyramid  follows  the  statements  made  by  recent 
workers  on  the  question  (Tourneux  and  Verdun) ;  His  has  claimed  that 
it  is  the  remains  of  the  stalk  connecting  the  thyreoid  with  the  floor  of  the 
pharynx,  and  which  he  terms  the  thyreo- glossal  duct. 

Two  other  pairs  of  bodies  enter  into  intimate  relations  with  the 


THE  BRANCHIAL   EPITHELIAL    BODIES 


297 


thm  IV 


thyreoid,  forming  what  have  been  termed  the  parathyreoid  bodies 
(Fig.  179,  pth1  and  pth2).  One  of  these  pairs  arises  as  a  thickening 
of  the  dorsal  portion  of  the  fourth  branchial  groove  and  the  other 
comes  from  the  corresponding 
portion  of  the  third  groove. 
The  members  of  the  former 
pair,  after  separating  from  their  pthm  IV 
points  of  origin,  come  to  lie  on 

the  dorsal  surface  of  the  lateral  sd  ^v^B  —  pthm  ill 

portions  of  the  thyreoid  body 
(Fig.  180,  pthm  IV)  in  close 
proximity  to  the  lateral  thy- 
reoids, while  those  of  the  other 
pair,  passing  further  backward, 
come  to  rest  behind  the  lower 
border  of  the  thyreoid  (Fig.  180, 
pthm  III).  The  cells  of  these 
bodies  do  not  become  divided 
into  cords  by  the  ingrowth  of 
connective  tissue  to  the  same 
extent  as  those  of  the  thyreoids, 
nor  do  they  become  separated 
into  follicles,  so  that  the  bodies 
are  readily  distinguishable  by 
their  structure  from  the  thy- 
reoid. 

From  the  ventral  portion  of 
the  third  branchial  groove  a 
pair  of  evaginations  develop, 
similar  to  those  which  produce 
the  lateral  thyreoids.  These  elongate  greatly,  and  growing  down- 
ward ventrally  to  the  thyreoid  and  separating  from  their  points  of  ori- 
gin, come  to  lie  below  the  thyreoids,  forming  the  thymus  gland  (Fig. 
179,  thy).  As  development  proceeds  they  pass  further  backward 
and  come  eventually  to  rest  upon  the  anterior  surface  of  the  peri- 


thm  HI 


Fig.  180. — Thyreoid,  Tyhmtjs  and 
Epithelial  Bodies  of  a  New-born 
Child. 

pthm  111  and  pthm  IV,  Para  thyreoids; 
sd,  thyreoid;  thm  III,  thymus;  thm  7T", 
lateral  thyreoid. — (Groschuff.) 


298 


THE    BRANCHIAL  EPITHELIAL   BODIES 


cardium.  The  cavity  which  they  at  first  contain  is  early  obliterated 
and  the  glands  assume  a  lobed  appearance  and  become  traversed  by 
trabecular  of  connective  tissue.  Lymphocytes,  derived,  according 
to  some  recent  observations,  directly  from  the  epithelium  of  the 
glands,  make  their  appearance  and  gradually  increase  in  number 
until  the  original  epithelial  cells  are  represented  only  by  a  number 
of  peculiar  spherical  structures,  consisting  of  cells  arranged  in  con- 
centric layers  and  known  as  Hassall's  corpuscles. 

The  glands  increase  in  size  until  about  the  fifteenth  year,  after 


Fig.  181. — Diagram  showing  the  Origin  of  the  Various  Branchial  Epithelial 

Bodies. 

Ith,  Lateral  thyreoids;  pp,  ultimobranchial  bodies;  pht1  and  phi2,  parathyreoids;  th, 
median  thyreoid;  thy,  thymus;  I  to  IV,  branchial  grooves. — (Kohn.) 

which  they  gradually  undergo  degeneration  into  a  mass  of  fibrous 
and  adipose  tissue. 

A  pair  of  evaginations  very  similar  to  those  that  give  rise  to  the 
thymus  are  also  formed  from  the  ventral  portion  of  the  fourth 
branchial  groove  (Figs.  179,  A  and  181,  lih).  As  a  rule  they  com- 
pletely disappear  in  later  stages  of  development,  but  occasionally 


THE    (ESOPHAGUS  299 

they  undergo  differentiation  into  small  masses  of  thymus-like  tissue, 
which  remain  associated  with  the  parathyreoids  from  the  same  arch 
(Fig.  180,  thm  IV).  They  have  been  termed  lateral  thyreoids,  but 
the  term  is  a  misnomer,  since  they  take  no  essential  part  in  the  for- 
mation of  the  thyreoid  body. 

Finally,  a  pair  of  outgrowths  arise  from  the  floor  of  the  pharynx 
just  behind  the  fifth  branchial  arch,  in  the  region  where  the  fifth 
groove,  if  developed,  would  occur.  These  ultimo-branchial  bodies, 
as  they  have  been  called,  usually  undergo  degeneration  at  an  early 
stage  and  disappear  completely,  though  occasionally  they  persist 
as  cystic  structures  embedded  in  the  substance  of  the  thyreoid. 

The  relation  of  these  various  structures  to  the  branchial  grooves  is 
shown  by  the  annexed  diagram  (Fig.  181),  and  from  it,  it  will  be  seen 
that  the  bodies  derived  from  the  third  and  fourth  grooves  are  serially 
equivalent.  Comparative  embryology  makes  this  fact  still  more  evident, 
since,  in  the  lower  vertebrates,  each  branchial  groove  contributes  to  the 
formation  of  the  thymus  gland.  The  terminology  used  above  for  the 
various  bodies  is  that  generally  applied  to  the  mammalian  organs,  but  it 
would  be  better,  for  the  sake  of  comparison  with  other  vertebrates,  to 
adopt  the  nomenclature  proposed  by  Groschuff,  who  terms  each  lateral 
thyreoid  a  thymus  IV,  while  each  thymus  lobe  is  a  thymus  III.  Similarly 
the  parathyreoids  are  termed  parathymus  III  and  IV,  the  term  thyreoid 
being  limited  to  the  median  thyreoid. 

The  Musculature  of  the  Pharynx. — The  pharynx  differs  from 
other  portions  of  the  archenteron  in  the  fact  that  its  walls  are  fur- 
nished with  voluntary  muscles,  the  principal  of  which  are  the  con- 
strictors and  the  stylo-pharyngeus.  This  peculiarity  arises  from 
the  relations  of  the  pharynx  to  the  branchial  arches.  It  has  been 
seen  that  in  the  higher  mammalia  the  dorsal  ends  of  the  third, 
fourth,  and  fifth  branchial  cartilages  disappear;  the  muscles  origin- 
ally associated  with  these  structures  persist,  however,  and  give  rise 
to  the  muscles  of  the  pharynx,  which  consequently  are  innervated 
by  the  ninth  and  tenth  nerves. 

The  Development  of  the  (Esophagus. — From  the  ventral 
side  of  the  lower  portion  of  the  pharynx  an  evagination  develops 
at  an  early  stage  which  is  destined  to  give  rise  to  the  organs  of 


3°° 


THE    STOMACH 


respiration;  the  development  of  this  may,  however,  be  conveniently- 
postponed  to  a  later  chapter  (Chapter  XII) . 

The  oesophagus  is  at  first  a  very  short  portion  of  the  archenteron 
(Fig.  182,  A),  but  as  the  heart  and  diaphragm  recede  into  the 
thorax,  it  elongates  (Fig.  182,  B)  until  it  eventually  forms  a  consider- 
able portion  of  the  digestive  tract.  Its  endodermal  lining,  like  that 
of  the  rest  of  the  digestive  tract  except  the  pharynx,  is  surrounded 


Fig.  182. — Reconstructions  of  the  Digestive  Tract  of  Embryos  of  (^4)  4.2  mm. 

and  (2?)  5  MM. 

all,  Allantois;  cl,  cloaca;  I,  lung;  li,  liver;  Rp,  Rathke's  pouch;  5,  stomach;  t,  tongue;  th> 

thyreoid  body;  Wd,  Wolffian  duct;  y,  yolk-stalk. — (His.) 

by  splanchnic  mesoderm  whose  cells  become  converted  into  non- 
striated  muscular  tissue,  which,  by  the  fourth  month,  has  separated 
into  an  inner  circular  and  an  outer  longitudinal  layer. 

The  Development  of  the  Stomach  and  Intestines. — By  the 
time  the  embryo  has  reached  a  length  of  about  5  mm.  its  constriction 


THE    INTESTINE  30I 

from  the  yolk-sac  has  proceeded  so  far  that  a  portion  of  the  digestive 
tract  anterior  to  the  yolk-sac  can  be  recognized  as  the  stomach  and 
a  portion  posterior  as  the  intestine.  As  first  the  stomach  is  a  simple, 
spindle-shaped  enlargement  (Fig.  182)  and  the  intestine  a  tube 
without  any  coils  or  bends,  but  since  in  later  stages  the  intestine 
grows  much  more  rapidly  in  length  than  the  abdominal  cavity,  a 
coiling  of  the  intestine  becomes  necessary. 

The  elongation  of  the  stomach  early  produces  changes  in  its 
position,  its  lower  end  bending  over  toward  the  right,  while  its  upper 
end,  owing  to  the  development  of  the  liver,  is  forced  somewhat 
toward  the  left.  At  the  same  time  the  entire  organ  undergoes  a 
rotation  about  its  longitudinal  axis  through  nearly  ninety  degrees, 
so  that,  as  the  result  of  the  combination  of  these  two  changes,  what 
was  originally  its  ventral  border  becomes  its  lesser  curvature  and 
what  was  originally  its  left  surface  becomes  its  ventral  surface. 

Hence  it  is  that  the  left  vagus  nerve  passes  over  the  ventral  and 
the  right  over  the  dorsal  surface  of  the  stomach  in  the  adult. 

In  the  meantime  the  elongation  of  the  oesophagus  has  carried 
the  stomach  further  away  from  the  lower  end  of  the  pharynx,  and 
from  being  spindle-shaped  it  has  become  more  pyriform,  as  in  the 
adult.  The  fundus,  it  may  be  noted,  is  not  due  to  a  general  en- 
largement of  the  organ  but  to  a  local  outpouching  of  the  upper 
dorsal  portion  of  its  wall. 

The  growth  of  the  intestine  results  in  its  being  thrown  into  a  loop 
opposite  the  point  where  the  yolk-stalk  is  still  connected  with  it, 
the  loop  projecting  ventrally  into  the  portion  of  the  ccelomic  cavity 
which  is  contained  within  the  umbilical  cord,  and  being  placed  so 
that  its  upper  limb  lies  to  the  right  of  the  lower  one.  Upon  the  latter 
a  slight  pouch-like  lateral  outgrowth  appears  which  is  the  beginning 
of  the  cacum  and  marks  the  line  of  union  of  the  future  small  and  large 
intestine.  The  small  intestine,  continuing  to  lengthen  more  rapidly 
than  the  large,  assumes  a  sinuous  course  (Fig.  183),  in  which  it  is 
possible  to  recognize  six  primary  coils  which  continue  to  be  recog- 
nizable until  advanced  stages  of  development  and  even  in  the  adult 
(Mall).     The  first  of  these  is  at  first  indistinguishable  from  the 


302 


THE    INTESTINE 


pyloric  portion  of  the  stomach  and  can  be  recognized  as  the  duo- 
denum only  by  the  fact  that  it  has  connected  with  it  the  ducts  of  the 
liver  and  pancreas;  as  development  proceeds,  however,  its  caliber 
diminishes  and  it  assumes  the  appearance  of  a  portion  of  the 
intestine. 

The   remaining   coils   elongate   rapidly   and   are   thrown  into 
numerous  secondary  coils,  all  of  which  are  still  contained  within  the 


Fig.  183.— Reconstruction  of  Embryo  of  20  mm. 
C,  Caecum;  K,  kidney; L,  liver;  S,  stomach;  SC,  suprarenal  bodies;  W,  mesonephros. — 

{Mall.) 

ccelom  of  theumbilical  cord  (Fig.  184).  When  the  embryo  has 
reached  a  length  of  about  40  mm.  the  coils  rather  suddenly  return 
to  the  abdominal  cavity,  and  now  the  caecum  is  thrown  over  toward 
the  right,  so  that  it  comes  to  lie  immediately  beneath  the  liver  on  the 
right  side  of  the  abdominal  cavity,  a  position  which  it  retains  until 
about  the  fourth  month  after  birth  (Treves).  The  portion  of  the 
large  intestine  which  formerly  projected  into  the  umbilical  ccelom  now 


THE    INTESTINE  303 

lies  transversely  across  the  upper  part  of  the  abdomen,  crossing  in 
front  of  the  duodenum  and  having  the  remaining  portion  of  the  small 
intestine  below  it.  The  elongation  continuing,  the  secondary  coils 
of  the  small  intestine  become  more  numerous  and  the  lower  portion 
of  the  large  intestine  is  thrown  into  a  loop  which  extends  trans- 
versely across  the  lower  part  of  the  abdominal  cavity  and  represents 
the  sigmoid  flexure  of  the  colon.  At  the  time  of  birth  this  portion 
of  the  large  intestine  is  relatively  much  longer  than  in  the  adult, 
amounting  to  nearly  half  the  entire  length  of  the  colon  (Treves), 
but  after  the  fourth  month  after  birth  a  readjustment  of  the  relative 


Fig.  184. — Reconstruction  of  the  Intestine  of  an  Embryo  of  19  mm.    The 
Figures  on  the  Intestine  Indicate  the  Primary  Coils. — {Mall.) 

lengths  of  the  parts  of  the  colon  occurs,  the  sigmoid  flexure  becoming 
shorter  and  the  rest  of  the  colon  proportionally  longer,  whereby  the 
caecum  is  pushed  downward  until  it  lies  in  the  right  iliac  fossa,  the 
ascending  colon  being  thus  established. 

When  this  condition  has  been  reached,  the  duodenum,  after 
passing  downward  for  a  short  distance  so  as  to  pass  dorsally  to  the 
transverse  colon,  bends  toward  the  left  and  the  secondary  coils 
derived  from  the  second  and  third  primary  coils  come  to  occupy 
the  left  upper  portion  of  the  abdominal  cavity.  Those  from  the 
fourth  primary  coil  pass  across  the  middle  line  and  occupy  the  right 


3°4 


THE    INTESTINE 


upper  part  of  the  abdomen,  those  from  the  fifth  cross  back  again  to 
the  left  lumbar  and  iliac  regions,  and  those  of  the  sixth  take  pos- 
session of  the  false  pelvis  and  the  right  iliac  region  (Fig.  185). 

Slight  variations  from  this  arrangement  are  not  infrequent,  but  it 
occurs  with  sufficient  frequency  to  be  regarded  as  the  normal.     A  failure 


Fig.  185. — Representation  of  the  Coilings  of  the  Intestine  in  the  Adult 
Condition.    The  Numbers  indicate  the  Primary  Coils. — (Mall.) 

in  the  readjustment  of  the  relative  lengths  of  the  different  parts  of  the 
colon  may  also  occasionally  occur,  in  which  case  the  caecum  will  retain  its 
embryonic  position  beneath  the  liver. 

The  yolk-stalk  is  continuous  with  the  intestine  at  the  extremity 
of  the  loop  which  extends  out  into  the  umbilical  coelom,  and  when  the 


THE    INTESTINE  305 

primary  coils  become  apparent  its  point  of  attachment  lies  in  the 
region  of  the  sixth  coil.  As  a  rule,  the  caliber  of  the  stalk  does  not 
increase  proportionally  with  that  of  the  intestine,  and  eventually 
its  embryomic  portion  disappears  completely.  Occasionally,  how- 
ever, this  portion  of  it  does  partake  of  the  increase  in  size  which 
occurs  in  the  intestine,  and  it  forms  a  blind  pouch  of  varying  length, 
known  as  Meckel's  diverticulum  (see  p.  113). 

The  ccecum  has  been  seen  to  arise  as  a  lateral  outgrowth  at  a 
time  when  the  intestine  is  first  drawn  out  into  the  umbilicus.  During 
subsequent  development  it  continues  to  in- 
crease in  size  until  it  forms  a  conical  pouch 
arising  from  the  colon  just  where  it  is  joined 
by  the  small  intestine  (Fig.  186).  The  en- 
largement of  its  terminal  portion  does  not  keep 
pace,  however,  with  that  of  the  portion  near- 
est the  intestine,  but  it  becomes  gradually 
more  and  more  marked  off  from  it  by  its  lesser 
caliber  and  gives  rise  to  the  vermiform  ap- 
pendix. At  birth  the  original  conical  form  Fig.  186.— caecum  of 
of  the  entire  outgrowth  is  still  quite  evident,        B  „  .      *°'3 

0  ^  c,  Colon;  1,  ileum. 

though  it  is  more  properly  described  as  funnel- 
shaped,  but  later  the  proximal  part,  continuing  to  increase  in  diam- 
eter at  the  same  rate  as  the  colon,  becomes  sharply  separated  from 
the  appendix,  forming  the  caecum  of  adult  anatomy. 

Up  to  the  time  when  the  embryo  has  reached  a  length  of  14  mm., 
the  inner  surface  of  the  intestine  is  quite  smooth,  but  when  a  length 
of  19  mm.  has  been  reached,  the  mucous  membrane  of  the  upper 
portion  becomes  thrown  into  longitudinal  folds,  and  later  these  make 
their  appearance  throughout  its  entire  length  (Fig.  187).  Later,  in 
embryos  of  60  mm.,  these  folds  break  up  into  numbers  of  conical 
processes,  the  villi,  which  increase  in  number  with  the  development 
of  the  intestine,  the  new  villi  appearing  in  the  intervals  between  those 
already  present.  Villi  are  formed  as  well  in  the  large  as  in  the  small 
intestine,  but  in  the  former  they  decrease  in  size  as  development 
proceeds   and  practically  disappear  toward  the  end  of  fetal  life. 


3°6 


THE    LIVER 


In  the  early  stages  the  endodermal  lining  of  the  digestive  tract  assumes 
a  considerable  thickness,  the  lumen  of  the  oesophagus  and  upper  part  of 
the  small  intestine  being  reduced  to  a  very  small  caliber.  In  later  stages 
a  rapid  increase  in  the  size  of  the  lumen  occurs,  apparently  associated 
with  the  formation  of  cavities  or  vacuoles  in  the  endodermal  epithelium. 
These  increase  in  size,  the  neighboring  cells  arrange  themselves  in  an 
epithelial  layer  around  their  walls  and  they  eventually  break  through  into 
the  general  lumen.  They  are  sometimes  sufficiently  large  to  give  the 
appearance  of  diverticula  of  the  gut,  but  later  they  flatten  out,  their 
cavities  becoming  portions  of  the  general  lumen. 

In  the  case  of  the  duodenum  the  thickening  of  the  endodermal 
lining  proceeds  to  such  an,  extent  that  in  embryos  of  from  12.5  mm.  to 
14.5  mm.  the  lumen  is  completely  obliterated  immediately  below  the 
opening  of  the  hepatic  and  pancreatic  ducts.  This  condition  is  interesting 
in  connection  with  the  occasional  occurrence  in  new-born  children  of  an 
atresia  of  the  duodenum.  Under  normal  conditions,  however,  the  lumen 
is  restored  by  the  process  of  vacuolization  described  above. 


Fig.  187. — Reconstruction  of  a  Portion  of  the  Intestine  of  an  Embryo  of  28 

mm.  showing  the  longitudinal  folds  from  which  the  villi  are  formed. 

{Berry.) 

The  Development  of  the  Liver. — The  liver  makes  its  appear- 
ance in  embryos  of  about  3  mm.  as  a  longitudinal  groove  upon  the 
ventral  surface  of  the  archenteron  just  below  the  stomach  and 
between  it  and  the  umbilicus.  The  endodermal  cells  lining  the 
anterior  portion  of  the  groove  early  undergo  a  rapid  proliferation, 
and  form  a  solid  mass  which  projects  ventrally  into  the  substance 


THE    LIVER 


3°7 


of  a  horizontal  shelf,  the  septum  transversum  (see  p.  318),  attached 
to  the  ventral  wall  of  the  body.  This  solid  mass  (Fig.  188,  L) 
forms  the  beginning  of  the  liver  proper,  while  the  lower  portion  of 
the  groove,  which  remains  hollow,  represents  the  future  gall-bladder 
(Fig.  188,  B).  Constrictions  appearing  between  the  intestine  and 
both  the  hepatic  and  cystic  portions  of  the  organ  gradually  separate 
these  from  the  intestine,  until  they  are  united  to  it  only  by  a  stalk 
which  represents  the  ductus  choledochus  (Fig.  188). 

The  further  development  of  the  liver,  so  far  as  its  external 


.     SS  2 


£---'* 


'    r 


Fig.  188. — Reconstruction  of  the  Liver  Outgrowths  of  Rabbit  Embryos  of 

(a)  5  mm.  and  (b)  of  8  mm. 

B,  Gall-bladder;  d,  duodenum;  DV,  ductus  venosus;L,  liver;  p,  dorsal  pancreas;  pm, 

ventral  pancreas;  rL,  right  lobe  of  the  liver;  S,  stomach. — (Hammar.) 


form  is  concerned,  consists  in  the  rapid  enlargement  of  the  hepatic 
portion  until  it  occupies  the  greater  part  of  the  upper  half  of  the 
abdominal  cavity,  its  ventral  edge  extending  as  far  down  as  the 
umbilicus.  In  the  rabbit  its  substance  becomes  divided  into  four 
lobes  corresponding  to  the  four  veins,  umbilical  and  vitelline,  which 
traverse  it,  and  the  same  condition  occurs  in  the  human  embryo, 
although  the  lobes  are  not  so  clearly  indicated  upon' the  surface  as  in 
the  rabbit.     The  two  vitelline  lobes  are  in  close  apposition  and  may 


3o8 


THE   LIVER 


almost  be  regarded  as  one,  a  median  ventral  lobe  which  embraces 
the  ductus  venosus  (Fig.  188,  B,  DV),  while  the  umbilical  lobes  are 
more  lateral  and  dorsal  and  represent  the  right  (rL)  and  left  lobes 
of  the  adult  liver.  The  remaining  definite  lobes,  the  caudate 
(Spigelian)  and  quadrate,  are  of  later  formation,  standing  in  relation 
to  the  vessels  which  cross  the  lower  surface  of  the  liver. 

The  ductus  choledochus  is  at  first  wide  and  short,  and  near  its 
proximal  end  gives  rise  to  a  small  outgrowth  on  each  side,  one  of 
which  becomes  the  ventral  pancreas  (Fig.  188,  B,  pm).  Later  the 
duct  elongates  and  becomes  more  slender,  and  the  gall-bladder  is 


Fig.  189. — Transverse  Section  through  the  Liver  oe  an  Embryo  of  Four 

Months. 
in,  Intestine;  I,  liver;  W,  Wolffian  body. — {Toldt  and  Zuckerkandl.) 

constricted  off  from  it,  the  connecting  stalk  becoming  the  cystic 
duct.  The  hepatic  ducts  are  apparently  developed  from  the  liver 
substance  and  are  relatively  late  in  appearing. 

Shortly  after  the  hepatic  portion  has  been  differentiated  its  sub- 
stance becomes  permeated  by  numerous  blood-vessels  (sinusoids) 
and  so  divided  into  anastomosing  trabeculae  (Fig.  189).  These  are 
at  first  irregular  in  size  and  shape,  but  later  they  become  more  slender 
and  more  regularly  cylindrical,  forming  what  have  been  termed  the 


THE    LIVER 


3°9 


hepatic  cylinders.  In  the  center  of  each  cylinder,  where  the  cells 
which  form  it  meet  together,  a  fine  canal  appears,  the  beginning  of 
a  bile  capillary,  the  cylinders  thus  becoming  converted  into  tubes 
with  fine  lumina.  This  occurs  at  about  the  fourth  week  of  develop- 
ment and  at  this  time  a  cross-section  of  a  cylinder  shows  it  to  be 
composed  of  about  three  or  four  hepatic  cells  (Fig.  190,  A),  among 
which  are  to  be  seen  groups  of  smaller  cells  (e)  which  are  erythro- 
cytes, the  liver  having  assumed  by  this  time  its  haematopoietic  func- 
tion (see  p.  225).     This  condition  of  affairs  persists  until  birth,  but 


Fig.  190.- — Transverse  Sections  of  Portions  of  the  Liver  of  (.4)  a  Fetus  of  Six 

Months  and  (B)  a  Child  of  Four  Years. 

be,  Bile  capillary;  e,  erythrocyte;  he,  hepatic  cylinder. — (Toldt  and  Zuckerkandl.) 


later  the  cylinders  undergo  an  elongation,  the  cells  of  which  they  are 
composed  slipping  over  one  another  apparently,  so  that  the  cylin- 
ders become  thinner  as  well  as  longer  and  show  for  the  most  part 
only  two  cells  in  a  transverse  section  (Fig.  190,  B);  and  in  still  later 
periods  the  two  cells,  instead  of  lying  opposite  one  another,  may 
alternate,  so  that  the  cylinders  become  even  more  slender. 

The  bile  capillaries  seem  to  make  their  appearance  first  in  cylin- 
ders which  lie  in  close  relation  to  branches  of  the  portal  vein  (Fig.  191) , 


3io 


THE    LIVER 


and  thence  extend  throughout  the  neighboring  cylinders,  anastomos- 
ing with  capillaries  developing  in  relation  to  neighboring  portal 
branches.  As  the  extension  so  proceeds  the  older  capillaries  con- 
tinue to  enlarge  and  later  become  transformed  into  bile-ducts  (Fig. 
191,  C),  the  cells  of  the  cylinders  in  which  these  capillaries  were 
situated  becoming  converted  into  the  epithelial  lining  of  the  ducts. 

The  lobules,  which  form  so  characteristic  a  feature  of  the  adult 
liver,  are  late  in  appearing,  not  being  fully  developed  until  some 
time  after  birth.  They  depend  upon  the  relative  arrangement  of 
the  branches  of  the  portal  and  hepatic  veins;  these  at  first  occupy 
distinct  territories  of  the  liver  substance,  being  separated  from  one 
another  by  practically  the  entire  thickness  of  the  liver,  although  of 


Fig.  191. — Injected  Bile  Capillaries  of  Pig  Embryos  of  (A)  8  cm.,  (B)  16  cm.,  and 
(C)  of  Adult  Pig. — (Hendrickson.) 

course  connected  by  the  sinusoidal  capillaries  which  lie  between  the 
hepatic  cylinders.  During  development  the  two  sets  of  branches 
extend  more  deeply  into  the  liver  substance,  each  invading  the 
territory  of  the  other,  but  they  can  readily  be  distinguished  from  one 
another  by  the  fact  that  the  portal  branches  are  enclosed  within  a 
sheath  of  connective  tissue  (Glisson's  capsule)  which  is  lacking  to 
the  hepatic  vessels.  At  about  the  time  of  birth  the  branches  of  the 
hepatic  veins  give  off  at  intervals  bunches  of  terminal  vessels,  around 
which  branches  of  the  portal  vein  arrange  themselves,  the  liver  tissue 
becoming  divided  up  into  a  number  of  areas  which  may  be  termed 


THE    PANCREAS  3II 

hepatic  islands,  each  of  which  is  surrounded  by  a  number  of  portal 
branches  and  contains  numerous  dichotomously  branching  hepatic 
terminals.  Later  the  portal  branches  sink  into  the  substance  of  the 
islands,  which  thus  become  lobed,  and  finally  the  sinking  in  extends 
so  far  that  the  original  island  becomes  separated  into  a  number  of 
smaller  areas  or  lobules,  each  containing,  as  a  rule,  a  single  hepatic 
terminal  (the  intralobular  vein)  and  being  surrounded  by  a  number 
of  portal  terminals  {interlobular  veins) ,  the  two  systems  being  united 
by  the  capillaries  which  separate  the  cylinders  contained  within  the 
area.  The  lobules  are  at  first  very  small,  but  later  they  increase  in 
size  by  the  extension  of  the  hepatic  cylinders. 

Frequently  in  the  human  liver  lobules  are  to  be  found  containing  two 
intralobular  veins,  a  condition  with  results  from  an  imperfect  subdivision 
of  a  lobe  of  the  original  hepatic  island. 

The  liver  early  assumes  a  relatively  large  size,  its  weight  at  one 
time  being  equal  to  that  of  the  rest  of  the  body,  and  though  in  later 
embryonic  stages  its  relative  size  diminishes,  yet  at  birth  it  is  still  a 
voluminous  organ,  occupying  the  greater  portion  of  the  upper  half  of 
the  abdominal  cavity  and  extending  far  over  into  the  left  hypo- 
chondrium.  Just  after  birth  there  is,  however,  a  cessation  of 
growth,  and  the  subsequent  increase  proceeds  at  a  much  slower  rate 
than  that  of  the  rest  of  the  body,  so  that  its  relative  size  bcomes 
still  more  diminished  (see  Chap.  XVII).  The  cessation  of  growth 
affects  principally  the  left  lobe  and  is  accompanied  by  an  actual 
degeneration  of  portions  of  the  liver  tissue,  the  cells  disappearing 
completely,  while  the  ducts  and  blood-vessels  originally  present 
persist,  the  former  constituting  the  vasa  aberrantia  of  adult  anatomy. 
These  are  usually  especially  noticeable  at  the  left  edge  of  the  liver, 
between  the  folds  of  the  left  lateral  ligament,  but  they  may  also  be 
found  along  the  line  of  the  vena  cava,  around  the  gall-bladder,  and 
in  the  region  of  the  left  longitudinal  fissure. 

The  Development  of  the  Pancreas. — The  pancreas  arises  a 
little  later  than  the  liver,  as  two  or  three  separate  outgrowths,  one 
from  the  dorsal  surface  of  the  duodenum  (Fig.  192,  DP)  usually  a 
little  above  the  liver  outgrowth,  and  one  or  two  from  the  lower  part 


312 


THE    PANCREAS 


of  the  common  bile-duct.  Of  the  latter  outgrowths,  that  upon  the 
left  side  (Vps)  may  be  wanting  and,  if  formed,  early  disappears, 
while  that  of  the  right  side  (Vpd)  continues  its  development  to  form 
what  has  been  termed  the  ventral  pancreas.  Both  this  and  the 
dorsal  pancreas  continue  to  elongate,  the  latter  lying  to  the  left  of 
^^  the  portal  vein,  while  the  former, 

at  first  situated  to  the  right  of 
the  vein,  later  grows  across  its 
ventral  surface  so  as  to  come  into 
contact  with  the  dorsal  gland, 
with  which  it  fuses  so  intimately 
that  no  separation  line  can  be 
distinguished.  The  body  and 
tail  of  the  adult  pancreas  rep- 
resent the  original  dorsal  out- 
growth, while  the  right  ventral 
pancreas  becomes  the  head. 

Both  the  dorsal  and  ventral 
outgrowths  early  become  lobed, 
and  the  lobes  becoming  second- 
arily lobed  and  this  lobation  re- 
peating itself  several  times,  the 
compound  tubular  structure  of 
the  adult  gland  is  acquired,  the 
very  numerous  terminal  lobules 
becoming  the  secreting  acini, 
while  the  remaining  portions 
become  the  ducts.  Of  the  prin- 
cipal ducts,  there  are  at  first  two; 
that  of  the  dorsal  pancreas,  the 
duct  of  Santorini,  opens  into  the  duodenum  on  its  dorsal  surface, 
while  that  of  the  ventral  outgrowth,  the  duct  of  Wirsung,  opens 
into  the  ductus  choledochus.  When  the  fusion  of  the  two  portions 
of  the  gland  occurs,  an  anastomosis  of  branches  of  the  two  ducts 
develops  and  the  proximal  portion  of  the  duct  of  Santorini  may 


Fig.  iq2. — Reconstruction  of  the 
Pancreatic  Outgrowths  of  an  Embryo 
of  7.5  MM. 

D,  Duodenum;  Dc,  ductus  communis 
choledochus;  DP,  dorsal  pancreas;  Vpd, 
and  Vps,  right  and  left  ventral  pancreas. 
—{Helly.) 


LITERATURE  313 

degenerate,  so  that  the  secretion  of  the  entire  gland  empties  into 
the  common  bile-duct  through  the  duct  of  Wirsung. 

In  the  connective  tissue  which  separates  the  lobules  of  the  gland, 
groups  of  cells  occur,  which  have  no  connection  with  the  ducts  of 
the  gland,  and  form  what  are  termed  the  areas  ofLangerhans.  They 
arise  by  a  differentiation  of  the  cells  which  form  the  original  pancre- 
atic outgrowths,  and  have  been  distinguished  in  the  dorsal  pancreas 
of  the  guinea-pig  while  it  is  still  a  solid  outgrowth.  They  gradually 
separate  from  the  remaining  cells  of  the  outgrowth  and  come  to  lie 
in  the  mesenchyme  of  the  gland  in  groups  into  which,  finally,  blood- 
vessels penetrate. 

LITERATURE. 

E.  T.  Bell:  "The  Development  of  the  Thymus,"  Amer.  Journ.  of  Anat.,  v,  1906. 
J.  M.  Berry:  "On  the  Development  of  the  Villi  of  the  Human  Intestine,"  Anat. 

Anzeiger,  xvi,  1900. 
L.   Bolk:  "Die  Entwicklungsgeschichte  der  menschlichen  Lippen,"   Anat.  Hefte. 

xliv,  1908. 
L.  Bolk:  "Ueber  die  Gaumenentwicklung  und  die  Bedeutung  der   oberen   Zahn- 

leiste  beim  Menschen,"  Zeit.fiir  Morphol.  und  Anthropol.,  xiv,  191 1. 
J.  Bracket:  "Recherches  sur  le  developpement  du  pancreas  et  du  foie,"  Journ.  de 

I' Anat.  et  de  la  Physiol.,  xxxii,  1896. 
O.  C.  Bradley:  "A  Contribution  to  the  Morphology  and  Development  of  the  Mam- 
malian Liver,"  Journ.  Anat.  and  Physiol.,  xliii,  1908. 
H.  M.  de  Burlet:  "Die  ausseren  Formverhaltnisse  der  Leber  beim  menschlichen 

Embryo,"  Morphol.  Jahrb.,  XLII,  1910. 
R.  V.  Chamberldst:  "On  the  Mode  of  Disappearance  of  the  Villi  from  the  Colon  of 

Mammals,"  Anat.  Record,  in,  1909. 
J.  H.  Chievitz:  "Beitrage  zur  Entwicklungsgeschichte  der  Speicheldrusen,"  Archiv 

fiir  Anat.  und  Physiol., Anat.  Abth.,  1885. 
H.  Fox:  "The  Pharyngeal  Pouches  and  Their  Derivatives  in  the  Mammalia,"  Amer. 

Journ.  Anat.,  vin,  1908. 
K.  Groschtjff:  "  Ueber  das  Vorkommen  eines  Thymussegmentes  der  vierten  Kiemen- 

tasche  beim  Menschen,"  Anat.  Anzeiger,  xvn,  1900. 
O.  Grosser:  "Zur  Kenntnis  des  ultimobranchialen  Korpers  beim  Menschen,"  Anat. 

Anzeiger,  xxxvn,  19 10. 
L.  Grunwald:  "Ein  Beitrag  zur  Entstehung  und  Bedeutung  der  Gaumenmandeln," 

Anat.  Anzeiger,  xxxvn,  1910. 
J.   A.   Hammar:   "Einige  Plattenmodelle  zur  Beleuchtung  der  friiheren   embryonal 

Leberentwicklung,"  Arch.f.  Anat.  undPhys.,  Anat.  Abth.,  1893. 


314  LITERATURE 

J.  A.  Hammar:  "Notiz  iiber  die  Entwicklung  der  Zunge  und  der  Mundspeicheldrtisen 

beim  Menschen,"  Anat.  Anzeiger,  xix,    1901. 
J.  A.  Hammar:  "Studien  iiber  die  Entwicklung  des  Vorderdarms  und  einiger  angren- 

zender  Organe,"  Arch./,  mikrosk.  Anat.,  lix  and  lx,  1902. 
K.  Helly:  "Zur  Entwickelungsgeschichte  der  Pancreasanlagen  und  Duodenalpapillen 

des  Menschen,"  Archivfiir  mikrosk.  Anat.,  lvi,  1900. 
K.  Helly:  "Studien  iiber  Langerhanssche  Inseln,"  Arch,  filr  mikrosk.  Anat.,fXMU, 

1907. 
W.  F.  Hendrickson:  "The  Development  of  the  Bile-capillaries  as  revealed  by  Golgi's 

Method,"  Johns  Hopkins  Hospital  Bulletin,  1898. 
W.  His:  "Anatomie  menschlicher  Embryonen,"  Leipzig,  1882-1886. 
F.  Hochstetter:   "Ueber  die  Bildung  der  primitiven  Choanen  beim  Menschen," 

Anat.  Anzeiger,  vii,  1892. 
N.  W.  Ingalls  :  "  A  Contribution  to  the  Embryology  of  the  Liver  and  Vascular  System 

in  Man,"  Anat.  Record,  II,  1908. 
C.  M.  Jackson:  "  On  the  Development  and  Topography  of  the  Thoracic  and  Abdominal 

Viscera,"  Anat.  Record,  in,  1909. 
F.  P.  Johnson:  "The  Development  of  the  Mucous  Membrane  of  the  (Esophagus, 

Stomach  and  Small  Intestine  in  the  Human  Embryo,"  Amer.  J  own.  Anat.,  x, 

1910. 

E.  Kallius:  "Beitrage  zur  Entwicklung  der  Zunge,  3teTh.  Saugetiere.  I.  Sus  scrofa," 

Anat.  Hefte,  xli,  1910. 

F.  Keibel:  "Zur  Entwickelungsgeschichte  des  menschlichen  Urogenital-apparatus," 

Archivfiir  Anat.  und  Physiol.,  Anat.  Abth.,  1896. 

G.  Killian:   "Ueber  die  Bursa  und  Tonsilla  pharyngea,"  Morphol.  Jahrbuch,  xiv, 

1888. 
A.  Kohn:  "Die  Epithelkorperchen,"  Ergebnisse  der  Anat.  und  Entwicklungsgesch.,  ix, 

1899. 
H.  Kuster:  "Zur  Entwicklungsgeschichte  der  Langerhans'schen  Inseln  im  Pancreas 

beim  menschlichen  Embryo,"  Arch,  filr  mikrosk.,  Anat.,  lxiv,  1904. 
F.  T.  Lewis  and  F.  W.  Thyng:  "The  Regular  Occurrence  of  Intestinal  Diverticula 

in  Embryos  of  the  Pig,  Rabbit  and  Man,"  Amer.  Journ.  Anat.,  vii,  1908. 
F.  P.  Mall:  "Ueber  die  Entwickelung  des  menschlichen  Darmes  und  seiner  Lage 

beim  Erwachsenen,"  Archivfiir  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,  1897. 
F.  P.  Mall:  "A  Study  of  the  Structural  Unit  of  the  Liver,"  Amer.  Journ.  of  Anat.,  V, 

1906. 
R.  Mayer:  "  Ueber  die  Bildung  des  Recessus  pharyngeus  medius  s.  Bursa  pharyngis  in 

zusammenhang  mit  der  Chorda  bei  menschlichen  Embryonen,"  Anat.  Anzeiger, 

xxxvii,  1910. 
J.  F.  Meckel:  "  Bildungsgeschichte  des  Darmkanals  der  Saugethiere  und  namentlich 

des  Menschen,"  Archivfiir  Anat.  und  Physiol.,  in,  1817. 
T.  Mironescu:  "  Ueber  die   Entwicklung  der   Langerhans'  schen   Inseln  bei  men- 
schlichen Embryonen,"  Arch,  fur  mikrosk.  Anat.,  lxxvi,  1911. 
W.  J.  Otis:  "  Die  Morphogenese  und  Histogenese  des  Analhockers  nebst  Bemerkungen 

iiber  die  Entwicklung  der  Sphincter  ani  externus  beim  Menschen,"  Anat.  Hefte, 

xxx,  1906. 


LITERATURE  315 

R.  M.  Pearce:  "The  Development  of  the  Islands  of  Langerhans  in  the  Human 

Embryo,"  Amer.  Journ.  of  Anal.,  11,  1902. 
C.  Rose:  "Ueber  die  Entwicklung  der  Zahne  des  Menschen,"  Archiv  fur  mikrosk. 

Anat.,  xxxviii,  1891. 
G.  Schorr:  "  Zur  Entwickelungsgeschichte  des  secundaren  Gaumens,"  Anat.  Hefte, 

xxxvi,  1908. 
G.  Schorr:  "Ueber  Wolfsrachen  von  Standpunkt  der  Embryologie  und  pathologischen 

Anatomie,"  Arch,  fur  palholog.  Anal.,  cxcvn,  1909. 
A,  Swaen:  "Recherches  sur  le  developement  du  foie,  du  tube  digestif,  de  l'arriere- 

cavite  du  peritoine  et  du  mesentere,"  Journ.  de  I' Anal,  et  de  la  Physiol.,  xxxii, 

1896,  and  xxxiii,  1897. 
J.  Tandler:  "Zur  Entwickelungsgeschichte  des  menschlichen  Duodenum  in  frtihen 

Embryonalstadien,"  Morphol.  Jahrbuch,  xxix,  1900. 
P.  Thompson:  "A  Note  on  the  Development  of  the  Septum  Transversum  and  the 

Liver,"  Journ.  Anat.  and  Phys.,  xlii,  1908. 
F.  W.  Thyng:  "Models  of  the  Pancreas  in  Embryos  of  the  Pig,  Rabbit,  Cat  and 

Man,"  Amer,  Journ.  Anat.,  vn,  1908. 
C.  Toldt  and  E.  Zuckerkandl:  "Ueber  die  Form  und  Texturveranderungen   der 

menschlichen  Leber  wahrend  des  Wachsthums,"  Sitzungsber.  der  kais.  Akad. 

Wissensch.  Wien.,  M ath.-N aturwiss .  Classe,  lxxii,  1875. 
F.  Tourneux  and  P.  Verdun:  "Sur  les  premiers  developpements  de  la  Thyroide,  du 

Thymus  et  des  glandes  parathyroidiennes  chez  l'homme,"  Journ.  de  I' Anat.  et 

de  la  Physiol.,  xxxiii,  1897. 
F.  Treves:  "Lectures  on  the  Anatomy  of  the  Intestinal  Canal  and  Peritoneum  in 

Man,"  British  Medical  Journal,  1,  1885. 


CHAPTER  XI 

THE  DEVELOPMENT  OF  THE  PERICARDIUM,  THE 
PLEURO-PERITONEUM  AND  THE  DIAPHRAGM. 

It  has  been  seen  (p.  229)  that  the  heart  makes  its  appearance  at 
a  stage  when  the  greater  portion  of  the  ventral  surface  of  the  intes- 
tine is  still  open  to  the  yolk-sac.  "The  ventral  mesoderm  splits  to 
form  the  somatic  and  splanchnic  layers  and  the  heart  develops  as  a 
fold  in  the  latter  on  each  side  of  the  median  line,  projecting  into  the 
ccelomic  cavity  enclosed  by  the  two  layers  (Fig.  136,  A).  As  the 
constriction  of  the  anterior  part  of  the  embryo  proceeds  the  two 
heart  folds  are  brought  nearer  together  and  later  meet,  so  that  the 
heart  becomes  a  cylindrical  structure  lying  in  the  median  line  of  the 
body  and  is  suspended  in  the  ccelom  by  a  ventral  band,  the  ventral 
tnesocardium,  composed  of  two  layers  of  splanchnic  mesoderm 
which  extend  to  it  from  the  ventral  wall  of  the  body,  and  by  a 
similar  band,  the  dorsal  tnesocardium,  which  unites  it  with  the 
splanchnic  mesoderm  surrounding  the  digestive  tract.  The  ven- 
tral mesocardium  soon  disappears  (Fig.  136  C)  and  the  dorsal  one 
also  vanishes  somewhat  later,  so  that  the  heart  comes  to  lie  freely 
in  the  ccelomic  cavity,  except  for  the  connections  which  it  makes 
with  the  body-walls  by  the  vessels  which  enter  and  arise  from  it. 

The  ccelomic  cavity  of  the  embryo  does  not  at  first  communicate 
with  the  extra-embryonic  ccelom,  which  is  formed  at  a  very  early 
period  (see  p.  67),  but  later  when  the  splitting  of  the  embryonic 
mesoderm  takes  place  the  two  cavities  become  continuous  behind 
the  heart,  but  not  anteriorly,  since  the  ventral  wall  of  the  body  is 
formed  in  the  heart  region  before  the  union  can  take  place.  It  is 
possible,  therefore,  to  recognize  two  portions  in  the  embryonic 
ccelom,  an  anterior  one,  the  parietal  cavity  (His),  which  is  never 
connected  laterally  with  the  extra-embryonic  cavity,  and  a  posterior 
one,  the  trunk  cavity,  which  is  so  connected.^The  heart  is  situated 

316 


THE    PERICARDIUM  AND    PLEURO-PERITONEUM 


3*7 


in  the  parietal  cavity,  a  considerable  portion  of  which  is  destined  to 
become  the  pericardial  cavity. 

Since  the  parietal  cavity  lies  immediately  anterior  to  the  still 
wide  yolk-stalk,  as  may  be  seen  from  the  position  of  the  heart  in  the 
embryo  shown  in  Fig.  53,  it  is  bounded 
posteriorly  by  the  yolkstalk.  This 
boundary  is  complete,  however,  only 
in  the  median  line,  the  cavity  being 
continuous  on  either  side  of  the  yolk- 
stalk with  the  trunk-cavity  by  pas- 
sages which  have  been  termed  the 
recessus  parietales  (Fig.  193,  Bp  and 
Rca).  Passing  forward  toward  the 
heart  in  the  splanchnic  mesoderm 
which  surrounds  the  yolkstalk  are  the 
large  vitelline  veins,  one  on  either  side, 
and  these  shortly  become  so  large  as 
to  bring  the  splanchnic  mesoderm  in 
which  they  lie  in  contact  with  the  so- 
matic mesoderm  which  forms  the  lat- 
eral wall  of  each  recess.  Fusion  of 
the  two  layers  of  mesoderm  along  the 
course  of  the  veins  now  takes  place, 
and  each  recess  thus  becomes  divided 
into  two  parallel  passages,  which  have 
been  termed  the  dorsal  (Fig.  194,  rpd) 
and  ventral  irpv)  parietal  recesses. 
Later  the  two  veins  fuse  in  the  upper 
portion  of  their  course  to  form  the  be- 
ginning of  the  sinus  venosus,  with  the  result  that  the  ventral  re- 
cesses become  closed  below  and  their  continuity  with  the  trunk- 
cavity  is  interrupted,  so  that  they  form  two  blind  pouches  extending 
downward  a  short  distance  from  the  ventral  portion  of  the  floor  of 
the  parietal  cavity.  The  dorsal  recesses,  however,  retain  their 
continuity  with  the  trunk-cavity  until  a  much  later  period. 


Om 


Rca 


Fig.  1 93 . — Reconstruction 
of  a  Rabbit  Embryo  of  Eight 
Days,  with  the  Pericardial 
Cavity  Laid  Open. 

A,  Auricle;  Aob,  aortic  bulb; 
A. V.,  atrio- ventricular  communi- 
cation; Bp,  ventral  parietal  re- 
cess; Om,  vitelline  vein;  Pc,  peri- 
cardial cavity;  Rca,  dorsal  pari- 
etal recess;  Sv,  sinus  venosus;  V, 
ventricle. — (His.) 


3^ 


THE   PERICARDIUM  AND   PLEURO-PERITONEUM 


By  the  fusion  of  the  vitelline  veins  mentioned  above,  there  is 
formed  a  thick  semilunar  fold  which  projects  horizontally  into  the 
ccelom  from  the  ventral  wall  of  the  body  and  forms  the  floor  of  the 
ventral  part  of  the  parietal  recess.  This  is  known  as  the  septum 
transversum,  and  besides  containing  the  anterior  portions  of  the 
vitelline  veins,  it  also  furnishes  a  passage  by  which  the  ductus 
Cuvieri,  formed  by  the  union  of  the  jugular  and  cardinal  veins, 
reach  the  heart.  Its  dorsal  edge  is  continuous  in  the  median  line 
with  the  mesoderm  surrounding  the  digestive  tract  just  opposite 
the  region  where  the  liver  outgrowth  will  form,  but  laterally  this 
edge  is  free  and  forms  the  ventral  walls  of  the  dorsal  parietal  recess. 
An  idea  of  the  relations  of  the  septum  at  this  stage  may  be  obtained 


V0771 


rpv 


Fig.  194. — Transverse  Sections  of  a  Rabbit  Embryo  showing  the  Division  of 

the  Parietal  Recesses  by  the  Vitelline  Veins. 

am,  Amnion;  rp,  parietal  recess;  rpd  and  rpv,  dorsal  and  ventral  divisions  of  the  parietal 

recess;  vom,  vitelline  vein. — (Ravn.) 

from  Fig  195,  which  represents  the  anterior  surface  of  the  septum, 
together  with  the  related  parts,  in  a  rabbit  embryo  of  nine  days. 

The  Separation  of  the  Pericardial  Cavity. — The  septum  trans- 
versum is  at  first  almost  horizontal,  but  later  it  becomes  decidedly 
oblique  in  position,  a  change  associated  with  the  backward  move- 
ment of  the  heart.  As  the  closure  of  the  ventral  wall  of  the  body 
extends  posteriorly  the  ventral  edge  of  the  septum  gradually  slips 
downward  upon  it,  while  the  dorsal  edge  is  held  in  its  former  posi- 
tion by  its  attachment  to  the  wall  of  the  digestive  tract  and  the 
ductus  Cuvieri.     The  anterior  surface  of  the  septum  thus  comes  to 


THE    PERICARDIUM  AND    PLEURO-PERITONEUM 


3J9 


look  ventrally  as  well  as  forward,  and  the  parietal  cavity,  having 
taken  up  into  itself  the  blind  pouches  which  represented  the  ventral 
recesses,  comes  to  lie  to  a  large  extent  ventral  to  the  posterior  recesses. 
As  may  be  seen  from  Fig.  195,  the  ductus  Cuvieri,  as  they  bend 
from  the  lateral  walls  of  the  body  into  the  free  edges  of  the  septum, 
form  a  marked  projection  which  diminishes  considerably  the  open- 
ing of  the  dorsal  recesses  into  the  parietal  cavity.     In  later  stages 


am 


Fig.  195. — Reconstruction  from  a  Rabbit  Embryo  of  Nine  Days  showing  the 

Septum  Transversum  from  Above. 

am,  Amnion;  at,  atrium;  dc,  ductus  Cuvieri;  rpd,  dorsal  parietal  recess. — (Ravn.) 

this  projection  increases  and  from  its  dorsal  edge  a  fold,  which 
may  be  regarded  as  a  continuation  of  the  free  edge  of  the  septum, 
projects  into  the  upper  portions  of  the  recesses  and  eventually  fuses 
with  the  median  portion  of  the  septum  attached  to  the  wall  of  the  gut. 
In  this  way  the  parietal  cavity  becomes  a  completely  closed  sac,  and 
is  henceforward  known  as  the  pericardial  cavity,  the  original  ccelom 


32° 


THE    DIAPHRAGM 


being  now  divided  into  two  portions,  (i)  the  pericardial,  and  (2)  the 
pleuro -peritoneal  cavities,  the  latter  consisting  of  the  abdominal 
ccelom  together  with  the  two  dorsal  parietal  recesses  which  have 
been  separated  from  the  pericardial  (parietal)  cavity  and  are  des- 
tined to  be  converted  into  the  pleural  cavities. 

The  Formation  of  the  Diaphragm. — It  is  to  be  remembered  that 
the  attachment  of  the  transverse  septum  to  the  ventral  wall  of  the 
digestive  tract  is  opposite  the  point  where  the  liver  outgrowth 
develops.     When,  therefore,  the   outgrowth   appears,  it  pushes  its 


Fig.  196, — Diagrams  of  (A)  a  Sagittal  Section  of  an  Embryo  showing  the 
Liver  Enclosed  within  the  Septum  Transversum;  (B)  a  Frontal  Section  of  the 
Same;  (C)  a  Frontal  Section  of  a  Later  Stage  when  the  Liver  has  Separated 
from  the  Diaphragm. 

All,  Allantois;  CI,  cloaca;  D,  diaphragm ;Li,  liver;Ls,  falciform  ligament  of  the  liver; 
M,  mesentery;  Mg,  mesogastrium;  Pc,  pericardium; S,  stomach;5T,  septum  transversum; 
U,  umbilicus. 

way  into  the  substance  of  the  septum,  which  thus  acquires  a  very 
considerable  thickness,  especially  toward  its  dorsal  edge,  and  it 
furthermore  becomes  differentiated  into  two  layers,  an  upper  one, 
which  forms  the  floor  of  the  ventral  portion  of  the  pericardial  cavity 
and  encloses  the  Cuvierian  ducts,  and  a  lower  one  which  contains  the 
liver.  The  upper  layer  is  comparatively  thin,  while  the  lower  forms 
the  greater  part  of  the  thickness  of  the  septum,  its  posterior  surface 
meeting  the  ventral  wall  of  the  abdomen  at  the  level  of  the  anterior 
margin  of  the  umbilicus  (Fig.  196,  A). 


THE    DIAPHRAGM  32 1 

In  later  stages  of  development  the  layer  containing  the  liver 
becomes  separated  from  the  upper  layer  by  two  grooves  which, 
appearing  at  the  sides  and  ventrally  immediately  over  the  liver 
(Fig.  196,  B),  gradually  deepen  toward  the  median  line  and  dorsally. 
These  grooves  do  not,  however,  quite  reach  the  median  line,  a  por- 
tion of  the  lower  layer  of  the  septum  being  left  in  this  region  as  a 
fold,  situated  in  the  sagittal  plane  of  the  body  and  attached  above 
to  the  posterior  surface  of  the  upper  layer  and  below  to  the  anterior 
surface  of  the  liver,  beyond  which  it  is  continued  down  the  ventral 
wall  of  the  abdomen  to  the  umbilicus  (Fig.  196,  C,Ls).  This  is  the 
falciform  ligament  of  the  liver  of  adult  anatomy,  and  in  the  free 
edge  of  its  prolongation  down  the  ventral  wall  of  the  abdomen  the 
umbilical  vein  passes  to  the  under  surface  of  the  liver,  while  the  free 
edge  of  that  portion  which  lies  between  the  liver  and  the  digestive 
tract  contains  the  vitelline  (portal)  vein,  the  common  bile-duct,  and 
the  hepatic  artery.  The  diagram  given  in  Fig.  196  will,  it  is  hoped, 
make  clear  the  mode  of  formation  and  the  relation  of  this  fold, 
which,  in  its  entirety,  constitutes  what  is  sometimes  termed  the 
ventral  mesentery. 

And  not  only  do  the  grooves  fail  to  unite  in  the  median  line,  but 
they  also  fail  to  completely  separate  the  liver  from  the  upper  layer 
of  the  septum  dorsally,  the  portion  of  the  lower  layer  which  persists 
in  this  region  forming  the  coronary  ligament  of  the  liver.  The 
portion  of  the  lower  layer  which  forms  the  roof  of  the  grooves  be- 
comes the  layer  of  peritoneum  covering  the  posterior  surface  of  the 
upper  layer  (which  represents  the  diaphragm),  while  the  portion 
which  remains  connected  with  the  liver  constitutes  its  peritoneal 
investment. 

I  In  the  meantime  changes  have  been  taking  place  in  the  upper 
layer  of  the  septum.  As  the  rotation  of  the  heart  occurs,  so  that  its 
atrial  portion  comes  to  lie  anterior  to  the  ventricle,  the  Cuvierian 
ducts  are  drawn  away  from  the  septum  and  penetrate  the  posterior 
wall  of  the  pericardium,  the  separation  being  assisted  by  the  con- 
tinued descent  of  the  attachment  of  the  edge  of  the  septum  to  the 
ventral  wall  of  the  body.     During  the  descent,  when  the  upper 


322 


THE    PLEURAE 


layer  of  the  septum  has  reached  the  level  of  the  fourth  cervical  seg- 
ment, portions  of  the  myotomes  of  that  segment  become  prolonged 
into  it  and  the  layer  assumes  the  characteristics  of  the  diaphragm, 
the  supply  of  whose  musculature  from  the  fourth  cervical  nerves  is 
thus  explained. 

The  Pleurce. — The  diaphragm  is  as  yet,  however,  incomplete 
dors  ally,  where  the  dorsal  parietal  recesses  are  still  in  continuity  with 
the  trunk-cavity.  With  the  increase  in  thickness  of  the  septum 
transversum,  these  recesses  have  acquired  a  considerable  length 
antero-posteriorly,  and  into  their  upper  portions  the  outgrowths 
from  the  lower  part  of  the  pharynx  which  form  the  lungs  (see  page 
331)  begin  to  project.  The  recesses  thus  become  transformed 
into  the  pleural  cavities,  and  as  the  diaphragm  continues  to  descend, 
slipping  down  the  ventral  wall  of  the  body  and  drawing  with  it  the 
pericardial  cavity,  the  latter  comes  to  lie  entirely  ventral  to  the  pleural 
cavities.  The  free  borders  of  the  diaphragm,  which  now  form  the 
ventral  boundaries  of  the  openings  by  which  the  pleural  and  peri- 
toneal cavities  communicate,  begin  to  approach  the  dorsal  wall  of 
the  body,  with  which  they  finally  unite  and  so  complete  the  separa- 
tion of  the  cavities.  The  pleural  cavities  continue  to  enlarge  after 
their  separation  and,  extending  laterally,  pass  between  the  peri- 
cardium and  the  lateral  walls  of  the  body  until  they  finally  almost 
completely  surround  the  pericardium.  The  intervals  between  the 
two  pleurae  form  what  are  termed  the  mediastina. 

The  downward  movement  of  the  septum  transversum  extends 
through  a  very  considerable  interval,  which  may  be  appreciated 
from  the  diagram  shown  in  Fig.  197.  From  this  it  may  be  seen 
that  in  early  embryos  the  septum  is  situated  just  in  front  of  the  first 
cervical  segment  and  that  it  lies  very  obliquely,  its  free  edge  being 
decidedly  posterior  to  its  ventral  attachment.  When  the  downward 
displacement  occurs,  the  ventral  edge  at  first  moves  more  rapidly 
than  the  dorsal,  and  soon  comes  to  lie  at  a  much  lower  level.  The 
backward  movement  continues  throughout  the  entire  length  of  the 
cervical  and  thoracic  regions,  and  when  the  level  of  the  tenth  tho- 
racic segment  is  reached  the  separation  of  the  pleural  and  peritoneal 


THE    PERITONEUM 


323 


'atxJidbJb 


1  Cuw%ea£ 


1  SaUai 


cavities  is  completed,  and  then  the  dorsal  edge  begins  to  descend 
more  rapidly  than  the  ventral,  so  that  the  diaphragm  again  becomes 
oblique  in  the  same  sense  as  in  the  beginning,  a  position  which  it 
retains  in  the  adult. 

The  Development  of  the  Peritoneum. — The  peritoneal  cavity  is 
developed  from  the  trunk-cavity  of  early  stages  and  is  at  first  in  free 
communication  on  all  sides  of  the- 
yolk-stalk  with  the  extra-embryonic 
ccelom.  As  the  ventral  wall  of  the 
body  develops  the  two  cavities  become 
more  and  more  separated,  and  with 
the  formation  of  the  umbilical  cord 
the  separation  is  complete.  Along 
the  middorsal  line  of  the  body  the 
archenteron  forms  a  projection  into 
the  cavity  and  later  moves  further  out 
from  the  body-wall  into  the  cavity, 
pushing  in  front  of  it  the  peritoneum, 
which  thus  comes  to  surround  the  in- 
testine, forming  its  serous  coat,  and 
from  it  is  continued  back  to  the  dorsal 
body- wall  forming  the  mesentery. 

It  has  already  been  seen  that  on 
the  separation  of  the  liver  from  the 
septum  transversum,  the  tissue  of  the 
latter  gives  rise  to  the  peritoneal 
covering  of  the  liver  and  of  the  pos- 
terior surface  of  the  diaphragm,  and  also  to  the  ventral  mesentery. 
When  the  separation  is  taking  place,  the  rotation  of  the  stomach  al- 
ready described  (p.  301)  occurs,  with  the  result  that  the  portion  of  the 
ventral  mesentery  which  stretches  between  the  lesser  curvature  of  the 
stomach  and  the  liver  shares  in  the  rotation  and  comes  to  lie  in  a  plane 
practically  at  right  angles  with  that  of  the  suspensory  ligament,  its  sur- 
faces looking  dorsally  and  ventrally  and  its  free  edge  being  directed 
toward  the  right.     This  portion  of  the  ventral  mesentery  forms 


Fig.  197.— Diagram  showing 
the  Position  of  the  Diaphragm 
in  Embryos  of  Different  Ages. 
—{M  all.) 


324  THE    PERITONEUM 

what  is  termed  the  lesser  omentum,  and  between  it  and  the  dorsal 
surface  of  the  stomach  as  the  ventral  boundaries,  and  the  dorsal 
wall  of  the  abdominal  cavity  dorsally,  there  is  a  cavity,  whose  floor 
is  formed  by  the  dorsal  mesentery  of  the  stomach,  the  mesogastrium, 
the  roof  by  the  under  surface  of  the  left  half  of  the  liver,  while  to  the 
right  it  communicates  with  the  general  peritoneal  cavity  dorsal  to 
the  free  edge  of  the  lesser  omentum.  This  cavity  is  known  as  the 
bursa  omentalis  (lesser  sac  of  the  peritoneum),  and  the  opening  into 
it  from  the  general  cavity  or  greater  sac  is  termed  the  epiploic  foramen 
(foramen  of  Winslow).  Later,  the  floor  of  the  lesser  sac  is  drawn 
downward  to  form  a  broad  sheet  of  peritoneum  lying  ventral  to  the 
coils  of  the  small  intestine  and  consisting  of  four  layers;  this  repre- 
sents the  great  omentum  of  adult  anatomy  (Fig.  201). 

Although  the  form  assumed  by  the  bursa  omentalis  is  associated 
with  the  rotation  of  the  stomach,  it  seems  probable  that  its  real 
origin  is  independent  of  that  process  (Broman).  The  subserous 
tissue  of  the  transverse  septum  is  at  first  thick  and  includes  not  only 
the  liver,  but  also  the  pancreas  and  the  portion  of  the  digestive  tract 
which  becomes  the  stomach  and  the  upper  part  of  the  duodenum 
(Fig.  196,  A).  The  shrinkage  of  this  tissue  by  which  these  organs 
become  separated  from  the  septum  cannot  take  place  evenly  on 
account  of  the  relations  which  the  organs  bear  to  one  another,  so 
that  on  the  right  side  certain  peritoneal  recesses  are  formed,  one 
between  the  right  lung  and  the  stomach,  a  second  between  the  liver 
and  the  stomach,  and  a  third  between  the  pancreas  and  the  same 
structure.  In  man  these  three  recesses  communicate  with  one 
another  to  form  the  primary  bursa  omentalis,  and  open  by  a  com- 
mon epiploic  foramen  into  the  general  peritoneal  cavity.  The  rota- 
tion of  the  stomach,  which  takes  place  later,  merely  serves  to  modify 
the  original  bursa. 

In  the  human  embryo  a  small  recess  also  forms  upon  the  left  side 
between  the  left  lung  and  the  stomach.  Later  it  separates  from  the  rest 
of  the  bursa  omentalis  and  passes  up  along  the  side  of  the  oesophagus, 
coming  to  lie  on  its  right  side  between  it  and  the  diaphragm.  It  gives  rise 
to  a  small  serous  sac  that  lies  beneath  the  infracardial  lobe  of  the  right 


THE    PERITONEUM 


325 


lung,  when  this  is  present,  and  hence  has  been  termed  the  infracardial 
bursa. 

Below  the  level  of  the  upper  part  of  the  duodenum  the  ventral 
mensentery  is  wanting;  only  the  dorsal  mesentery  occurs.  So  long 
as  the  intestine  is  a  straight  tube  the  length  of  the  intestinal  edge  of 
this  mesentery  is  practically  equal  to  that  of  its  dorsal  attached  edge. 
The  intestine,  however,  increasing  in  length  much  more  rapidly 
than  the  abdominal  walls,  the  intestinal  edge  of  the  mesentery  soon 
becomes  very  much  longer  than  the  at- 
tached edge,  and  when  the  intestine  grows 
out  into  the  umbilical  ccelom  the  mesentery 
accompanies  it  (Fig.  198).  As  the  coils  of 
the  intestine  develop,  the  intestinal  edge  of 
the  mesentery  is  thrown,  into  corresponding 
folds,  and  on  the  return  of  the  intestine  to 
the  abdominal  cavity  the  mesentery  is 
thrown  into  a  somewhat  funnel-like  form 
by  the  twisting  of  the  intestine  to  form  its 
primary  loop  (Fig.  199).  All  that  portion 
of  the  mesentery  which  is  attached  to  the 
part  of  the  intestine  which  will  later  become 
the  jejunum,  ileum,  ascending  and  trans- 
verse colon,  is  attached  to  the  body-wall 
at  the  apex  of  the  funnel,  at  a  point  which  bryo  of  Six  Weeks. 
lies  to  the  left  of  the  duodenum.  Sp%^-VoMn°m^'' 

Up  to  this  stage  or  to  about  the  middle 
of  the  fourth  month  the  mesentery  has  retained  its  attachment  to  the 
median  line  of  the  dorsal  wall  of  the  abdomen  throughout  its  entire 
length,  but  later  fusions  of  certain  portions  occur,  whereby  the  orig- 
inal condition  is  greatly  modified.  One  of  the  earliest  of  these  fusions 
takes  place  at  the  apex  of  the  funnel,  where  the  portion  of  the  mesen- 
tery which  passes  to  the  tranverse  colon  and  arches  over  the  duo- 
denum fuses  with  the  ventral  surface  of  the  latter  portion  of  the 
intestine  and  also  with  the  peritoneum  covering  the  dorsal  wall  of  the 
abdomen  both  to  the  right  and  to  the  left  of  the  duodenum.     In  this 


Fig.  198. — Diagram 
showing  the  arrangement 
of  the  Mesentery  and  Vis- 
ceral Branches  of  the  Ab- 
dominal Aorta  in  an  Em- 


326 


THE    PERITONEUM 


way  the  attachment  of  the  transverse  mesocolon  takes  the  form  of  a 
transverse  line  instead  of  a  point,  and  this  portion  of  the  mesentery- 
divides  the  abdominal  cavity  into  two  portions,  the  upper  (anterior) 
of  which  contains  the  liver  and  stomach,  while  the  lower  contains 
the  remainder  of  the  digestive  tract  with  the  exception  of  the  duo- 
denum. By  passing  across  the  ventral  surface  of  the  duodenum 
and  fusing  with  it,  the  transverse  mesocolon  forces  that  portion  of 
the  intestine  against  the  dorsal  wall  of  the  abdomen  and  fixes  it  in 
that  position,  and  its  mesentery  thereupon  degenerates,  becoming 


md' 


Fig.  199.- 


-Diagrams  Illustrating  the  Development  of  the  Great  Omentum 

and  the  Transverse  Mesocolon. 

bid,  Caecum;  dd,  small  intestine;  dg,  yolk-stalk;  di,  colon;  du,  duodenum;  gc,  greater 

curvature  of  stomach;  gg,  bile  duct;  gn,  mesogastrium;  k,  point  where  the  loops  of  the 

intestine  cross;  mc,  mesocolon;  md,  rectum;  mes,  mesentery;  wf,  vermiform  appendix. 

— (Hertwig.) 

subserous  areolar  tissue,  the  duodenum  assuming  the   retroperito- 
neal position  which  characterizes  it  in  the  adult. 

The  descending  colon,  which  on  account  of  the  width  of  its  mes- 
entery is  at  first  freely  movable,  lies  well  over  to  the  left  side  of  the 
abdominal  cavity,  and  in  consequence  the  left  layer  of  its  mesentery 
lies  in  contact  with  the  parietal  layer  of  the  peritoneum.  A  fusion 
of  these  two  layers,  beginning  near  the  middle  line  and  thence  extend- 
ing outward,  takes  place,  the  fused  layers  becoming  converted  into 


THE    PERITONEUM 


327 


connective  tissue,  and  this  portion  of  the  colon  thus  loses  its  mesen- 
tery and  becomes  fixed  to  the  abdominal  wall.  The  process  by 
which  the  fixation  is  accomplished  may  be  understood  from  the 
diagrams  which  constitute  Fig.  200.  When  the  ascending  colon  is 
formed,  its  mesentery  undergoes  a  similar  fusion,  and  it  also  becomes 
fixed  to  the  abdominal  wall. 

The  fusion  of  the  mesentery  of  the  ascending  and  descending  colon 
remains  incomplete  in  a  considerable  number  of  cases  (one-fourth  to  one- 
third  of  all  cases  examined),  and  in  these  the  colons  are  not  perfectly 
fixed  to  the  abdominal  wall.  It  may  also  be  pointed  out  that  the  caecum 
and  appendix,  being  primarily  a  lateral  outpouching  of  the  intestine,  do 


Fig.  200— Diagrams  Illustrating  the  Manner  in  Which  the  Fixation  of  the 
Descending  Colon  (C)  takes  Place. 

not  possess  any  true  mesentery,  but  are  completely  enclosed  by  peritoneum. 
Usually  a  falciform  fold  of  peritoneum  may  be  found  extending  along  one 
surface  of  the  appendix  to  become  continuous  with  the  left  layer  of  the 
mesentery  of  the  ileum.  This,  however,  is  not  a  true  mesentery,  and  is 
better  spoken  of  as  a  mesenteriole. 

One  other  fusion  is  still  necessary  before  the  adult  condition  of 
the  mesentery  is  acquired.  The  great  omentum  consists  of  two 
folds  of  peritoneum  which  start  from  the  greater  curvature  of  the 
stomach  and  pass  downward  to  be  reflected  up  again  to  the  dorsal 
wall  of  the  abdomen,  which  they  reach  just  anterior  to  (above)  the 
line  of  attachment  of  the  transverse  mesocolon  (Fig.  201,  A).     At 


328 


THE    PERITONEUM 


first  the  attachment  of  the  omentum  is  vertical,  since  it  represents 
the  mesogastrium,  but  later,  by  fusion  with  the  parietal  peritoneum, 
it  assumes  a  transverse  direction,  while  at  the  same  time  the  pancreas, 
which  originally  lay  between  the  two  folds  of  the  mesogastrium,  is 
carried  dorsally  and  comes  to  have  a  retroperitoneal  position  in  the 
line  of  attachment  of  the  omentum.  By  this  change  the  lower  layer 
of  the  omentum  is  brought  in  contact  with  the  upper  layer  of  the 


Fig.  201. — Diagrams  showing  the  Development  of  the  Great  Omentum  and  its 
Fusion  with  the  Transverse  Mesocolon. 

B,  Bladder;  c,  transverse  colon;  d,  duodenum;  Li,  liver;  p,  pancreas;  R,  rectum;  S, 
stomach;  U,  uterus. — {After  Allen  Thomson.) 

transverse  mesocolon  and  a  fusion  and  degeneration  of  the  two  re- 
sults (Fig.  201  B),  a  condition  which  brings  it  about  that  the  omen- 
tum seems  to  be  attached  to  the  transverse  colon  and  that  the  pan- 
creas seems  to  lie  in  the  line  of  attachment  of  the  transverse  meso- 
colon. This  mesentery,  as  is  occurs  in  the  adult,  really  consists 
partly  of  a  portion  of  the  original  transverse  mesocolon  and  partly 
of  a  layer  of  the  great  omentum. 


LITERATURE  329 

By  these  various  changes  the  line  of  attachment  of  the  mesen- 
tery to  the  dorsal  wall  of  the  body  has  become  somewhat  compli- 
cated and  has  departed  to  a  very  considerable  extent  from  its  origi- 
nal simple  vertical  arrangement.  If  all  the  viscera  be  removed 
from  the  body  of  an  adult  and  the  mesentery  be  cut  close  to  the  line 
of  its  attachment,  the  course  of  the  latter  will  be  seen  to  be  as  fol- 
lows: Descending  from  the  under  surface  of  the  diaphragm  are 
the  lines  of  attachment  of  the  suspensory  ligament,  which  on 
reaching  the  liver  spread  out  to  become  the  coronary  and  lateral 
ligaments  of  that  organ.  At  about  the  mid-dorsal  line  these  lines 
become  continuous  with  those  of  the  mesogastriumr  which  curve 
downward  toward  the  left  and  are  continued  into  the  transverse  lines 
of  the  transverse  mesocolon.  Between  these  last,  in  a  slight  prolonga- 
tion, there  may  be  seen  to  the  right  the  cut  end  of  the  first  portion 
of  the  duodenum  as  it  passes  back  to  the  dorsal  wall  of  the  abdomen, 
and  at  about  the  mid-dorsal  line  the  cut  ends  of  its  last  part  become 
visible  as  it  passes  ventrally  again  to  become  the  jejunum.  From  the 
transverse  mesocolon  three  lines  of  attachment  pass  downward;  the 
two  lateral  broad  ones  represent  the  lines  of  fixation  of  the  ascending 
and  descending  colons,  while  the  narrower  median  one,  which 
curves  to  the  right,  represents  the  attachment  of  the  mesentery  of 
the  small  intestine  other  than  the  duodenum.  Finally,  from  the 
lower  end  of  the  fixation  line  of  the  descending  colon  the  mesentery 
of  the  sigmoid  is  continued  downward. 

The  special  developments  of  the  peritoneum  in  connection  with 
the  genito-urinary  apparatuus  will  be  considered  in  Chapter  XIII. 

LITERATURE. 

I.    Broman:    "Ueber   die  Entwicklung  und   Bedeutung   der  Mesenterial   und   der 

Korperhohlen  bei  den  Wirbeltieren,"  Ergebn.  der  Anat.  u.  Entw.,  XV,  1906. 
A.  Bracket:  "Die  Entwickelung  der  grossen  Korperhohlen  und  ihre  Trennung  von 

Einander,"  Ergebnisse  der  Anat.  und  Eniwickelungsgesch.,  vn,  1898. 
W.   His:   " Mittheilungen   zur  Embryologie   der   Saugethiere   und   des   Menschen," 

Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1881. 
F.  P.  Mall:  "Development  of  the  Human  Ccelom,"  Journal  of  Morphol.,  xii,  1897. 
F.  P.  Mall:  "On  the  Development  of  the  Human  Diaphragm,"  Johns  Hopkins 

Hospital  Bull.,  xii,  1901. 


330  LITERATURE 

E.  Ravn:  "Ueber  die  Bildung  der  Scheidewand  zwischen  Brust-  und  Bauchhohle  in 

Saugethierembryonen,"  Archiv  fur  Anat.  und  Physiol.,  Anat,  Abth.,  1889. 
A.  Swaen:  "Recherches  sur  le  developpement  du  foie,  du  tube  digestif,  de  l'arriere- 

cavite  du  peritoine  et  du  mesentere,"  Journ.  de  I' Anat.  et  de  la  Physiol.,  xxxii, 

1896;  xxxni,  1897. 
C.   Toldt:    "Bau   und   Wachstumsveranderungen   der   Gekrose   des   menschlichen 

Darmkanals,"   Denkschr.   der  kais.   Akad.   Wissensch.    Wien,   Math.-Naturwiss. 

Classe,  xli,  1879. 
C.   Toldt:   "Die  Darmgekrose  und  Netze  im  gesetzmassigen  und  gesetzwidrigen 

Zustand,"  Denkschr.  der  kais.  Akad.  Wissensch.  Wien.  Math.-Naturwiss.  Classe, 

lvt,  1889. 

F.  Treves:  "Lectures  on  the  Anatomy  of  the  Intestinal  Canal   and   Peritoneum," 

British  Medical  Journal,  I,  1885. 


CHAPTER  XII. 


THE  DEVELOPMENT  OF  THE  ORGANS  OF  RESPIRATION. 


The  Development  of  the  Lungs. — The  first  indication  of  the 
lungs  and  trachea  is  found  in  embryos  of  about  3.2  mm.  in  the 
form  of  a  groove  on  the  ventral  surface  of  the  oesophagus,  at  first  ex- 
tending almost  the  entire  length  of  that  portion  of  the  digestive 
tract.  As  the  oesophagus  lengthens  the  lung  groove  remains  con- 
nected with  its  upper  portion  (Fig.  182,  A),  and  furrows  which  ap- 
pear along  the  line  of  junction  of  the  groove  and  the  oesophagus 
gradually  deepen  and  separate 
the  two  structures  (Fig.  182,  B). 
The  separation  takes  place  earliest 
at  the  lower  end  of  the  groove 
and  thence  extends  upward,  so 
that  the  groove  is  transformed 
into  a  cylindrical  pouch  lying  ven- 
tral to  the  oesophagus  and  dorsal 
to  the  heart  and  opening  with  the 
oesophagus  into  the  terminal  por- 
tion of  the  pharynx. 

Soon  after  the  separation  of 
the  groove  from  the  oesophagus 
its  lower  end  becomes  enlarged 
and  bilobed,  and  since  this  lower 
end   lies,  with  the  oesophagus,  in 

the  median  attached  portion  of  the  dorsal  edge  of  the  septum  trans- 
versum,  the  lobes,  as  they  enlarge,  project  into  the  dorsal  parietal 
recesses  (Fig.  202),  and  so  become  enclosed  within  the  peritoneal 
lining  of  the  recesses  which  later  become  the  pleural  cavities. 

The  lobes,  which  represent  the  lungs,  do  not  long  remain  simple, 

33i 


RP 


Fig.  202. — Portion    of  a  Section 

THROUGH  AN  EMBRYO  OF  THE   FOURTH 

Week. 

A,  Aorta;  DC,  ductus  Cuvieri;  L, 
lung;  O,  oesophagus;  RP,  parietal  re- 
cess; VOm,  vitelline  vein. — (Toldt.) 


332 


THE    LUNGS 


but  bud-like  processes  arise  from  their  cavities,  three  appearing  in 
the  right  lobe  and  two  in  the  left  (Fig.  203,  A),  and  as  these  increase 
in  size  and  give  rise  to  additional  outgrowths,  the  structure  of  the 
lobes  rapidly  becomes  complicated  (Fig.  203,  B  and  C). 

The  lower  primary  process  on  each  side  may  be  regarded  as  a 
prolongation  of  the  bronchus,  while  the  remaining  process  or  pro- 
cesses represent  lateral  outgrowths  from  it.  Considerable  difference 
of  opinion  has  existed  as  to  the  nature  of  the  further  branching  of  the 
bronchi,  some  authors  regarding  it  as  a  succession  of  dichotomies, 
one  branch  of  each  of  these  placing  itself  so  as  to  be  in  the  line  of  the 


/\ 


Vp 

\ 


V 


c 


Fig.  203. — Reconstruction  of  the  Lung  Outgrowths  of  Embryos  of  (/I)  4.3, 

(5)  8.5,  and  (C)  10.5  MM. 

Ap,  Pulmonary  artery;  Ep,  eparterial  bronchus;  Vp,  pulmonary  vein;  7,  second  lateral 

bronchus;  II,  main  bronchi. — (His.) 

original  main  bronchus,  while  the  other  comes  to  resemble  a  lateral 
outgrowth,  and  other  observers  have  held  that  the  main  bronchus 
has  an  uninterrupted  growth,  all  other  branches  being  lateral  out- 
growths from  it,  and  the  branching  therefore  a  monopodial  process. 
The  recent  thorough  study  by  Flint  of  the  development  of  the  lung  of 
the  pig  shows  that,  in  that  form  at  least,  the  branching  is  a  mono- 
podial one,  and  that  from  the  main  bronchus  as  it  elongates  four  sets 
of  secondary  outgrowths  develop,  namely,  a  strong  lateral,  a  dorsal, 
a  ventral,  and  a  weak  and  variable  medial  set. 


THE   LUNGS 


333 


There  is  a  general  tendency  for  the  individual  branches  of  the 
various  sets  to  be  arranged  in  regular  succession  and  for  their  develop- 
ment to  be  symmetrical  in  the  two  lungs.  But  on  account  of  the 
necessity  under  which  the  lungs  are  placed  of  adapting  themselves 
to  the  neighboring  structures  and  at  the  same  time  affording  a 
respiratory  surface  as  large  as  possible,  an  amount  of  asymmetry 
supervenes.  Thus,  it  has  already  been  noted  that  in  the  earliest 
branching  a  single  lateral  bronchus  is  formed  in  the  left  lung  and  two 
in  the  right.  The  uppermost  of  these 
latter,  the  first  lateral  bronchus,  is  un- 
represented in  the  left  lung,  and  is  pecu- 
liar in  that  it  lies  behind  the  right  pul- 
monary artery  (Fig.  203,  C),  or  in  the 
adult,  after  the  recession  of  the  heart, 
above  it,  whence  it  is  termed  the  epar- 
terial  bronchus.  Its  absence  on  the  left 
side  is  perhaps  due  to  its  suppression  to 
permit  the  normal  recession  of  the  aortic 
arch  (Flint). 

So,  too,  the  inclination  of  the  heart 
causes  a  suppression  of  the  second  ven- 
tral bronchus  in  the  left  lung,  but  at 
the  same  time  it  affords  opportunity  for 
an  excessive  development  of  the  corre- 
sponding bronchus  of  the  right  lung, 
which  pushes  its  way  between  the  heart 
and  the  diaphragm  and  is  known  as  the 
infra-cardiac  bronchus. 

As  soon  as  the  unpaired  first  lateral  bronchus  and  the  paired  sec- 
ond lateral  bronchi  are  formed  mesenchyme  begins  to  collect  around 
each  of  them  and  also  around  the  main  bronchi,  the  lobes  of  the 
adult  lung,  three  in  the  right  lung  and  two  in  the  left,  being  thus 
outlined.  A  development  of  mesenchyme  also  takes  place  around 
the  excessively  developed  right  second  ventral  bronchus,  and  some- 
times produces  a  well-marked  infra-cardiac  lobe  in  the  right  lung. 


Fig.  204. — Diagram  of  the 
Final  Branches  of  the  Mam- 
malian Bronchi. 

A,  Atrium;  B,  bronchus;  S, 
air-sac. — (Miller.) 


334 


THE    LARYNX 


In  later  stages  the  various  bronchi  of  each  lobe  give  rise  to 
additional  branches  and  these  again  to  others,  and  the  mesenchyme 
of  each  lobe  grows  in  between  the  various  branches.  At  first  the 
amount  of  mesenchyme  separating  the  branches  is  comparatively 
great,  but  as  the  branches  continue,  the  growth  of  the  mesenchyme 
fails  to  keep  pace  with  it,  so  that  in  later  stages  the  terminal  enlarge- 
ments are  separated  from  one  another  by  only  very  thin  partitions 
of  mesenchyme,  in  which  the  pulmonary  vessels  form  a  dense  net- 
work. The  final  branching  of  each  ultimate  bronchus  or  bronchiole 
results  in  the  formation  at  its  extremity  of  from  three  to  five  enlarge- 
ments, the  atria  (Fig.  204,  A),  from  which  arise  a  number  of  air-sacs 
(S)  whose  walls  are  pouched  out  into  slight  diverticula,  the  air-cells 
or  alveoli.     Such  a  combination  of  atria,   air-sacs,   and  air-cells 

constitutes  a  lobule,  and  each  lung 
is  composed  of  a  large  number  of 
such  units. 

The  greater  part  of  the  origi- 
nal   pulmonary   groove    becomes 
converted  into  the  trachea,  and  in 
the   mesenchyme   surrounding   it 
the  incomplete  cartilaginous  rings 
develop    at   about  the  eighth  or 
ninth  week.     The  cells  of  the  epi- 
thelial lining  of  the  trachea  and 
bronchi  remain  columnar  or  cu- 
bical in  form  and  become  ciliated 
at  about  the  fourth  month,   but 
those  of  the  epithelium  of  the  air- 
sacs  become  greatly  flattened  and 
constitute     an    exceedingly    thin 
layer  of  pavement  epithelium. 
The  Development  of  the  Larynx. — The  opening  of  the  upper 
end  of  the  pulmonary  groove  into  the  pharynx  is  situated  at  first 
just  behind  the  fourth  branchial  furrow  and  is  surrounded  anteriorly 
and  laterally  by  the  PI  -shaped  ridge  already  described  (p.  294)  as 


Fig.  205. — Reconstruction  of  the 
Opening  into  the  Larynx  in  an  Em- 
bryo of  Twenty-eight  Days,  Seen 
from  Behind  and  Above,  the  Dorsal 
Wall  of  the  Pharynx  being  Cut 
Away. 

co,  Cornicular,  and  cu,  cuneiform  tu- 
bercle; Ep,  epiglottis;  T,  unpaired  por- 
tion of  the  tongue. — (Kallius.) 


THE    LARYNX  335 

the  furcula,  this  separating  it  from  the  posterior  portion  of  the 
tongue  (Fig.  178).  The  anterior  portion  of  this  ridge,  which  is 
apparently  derived  from  the  ventral  portions  of  the  third  branchial 
arch,  gradually  increases  in  height  and  forms  the  epiglottis,  while 
the  lateral  portions,  which  pass  posteriorly  into  the  margins  of  the 
pulmonary  groove,  form  the  ary epiglottic  folds.  When  the  pulmon- 
ary groove  separates  from  the  oesophagus,  the  opening  of  the  trachea 
into  the  pharynx  is  somewhat  slit-like  and  is  bounded  laterally  by 
the  aryepiglottic  folds,  whose  margins  present  two  elevations  which 
may  be  termed  the  comicular  and  cuneiform  tubercles  (Fig.  205,  co 
and  cu,  and  Fig.  175).  The  opening  is,  however,  for  a  time  almost 
obliterated  by  a  thickening  of  the  epithelium  covering  the  ridges, 


Fig.  206.— Reconstruction  of  the  Mesenchyme  Condensations  which  Represent 

the  Hyoid  and  Thyreoid  Carthages  in  an  Embryo  of  Forty  Days. 

The  darkly  shaded  areas  represent  centers  of  chondrification.     c.ma,  Greater  cornu  of 

hyoid;  c.mi,  lesser  cornu;  Th,  thyreoid  cartilage. — (Kallius.) 

and  it  is  not  until  the  tenth  or  eleventh  week  of  development  that 
it  is  re-established.  Later  than  this,  at  the  middle  of  the  fourth 
month,  a  linear  depression  makes  its  appearance  on  the  mesial 
surface  of  each  ary-epiglottic  fold,  forming  the  beginning  of  the 
ventricle,  and  although  at  first  the  depression  lies  horizontally,  its 
lateral  edge  later  bends  anteriorly,  so  that  its  surfaces  look  outward 
and  inward.  The  lips  which  bound  the  opening  of  the  ventricle 
into  the  laryngeal  cavity  give  rise  to  the  ventricular  and  vocal  folds. 
The  cartilages  of  the  larynx  can  be  distinguished  during  the 
seventh  week  as  condensations  of  mesenchyme  which  are  but 
indistinctly  separated  from  one  another.  The  thyreoid  cartilage  is 
represented  at  this  stage  by  two  lateral  plates  of  mesenchyme, 


336  THE    LARYNX 

separated  from  one  another  both  ventrally  and  dorsally,  and  each 
of  these  plates  undergoes  chondrification  from  two  separate  centers 
(Fig.  206) .  These,  as  they  increase  in  size,  unite  together  and  send 
prolongations  ventrally  which  meet  in  the  mid-ventral  line  with  the 
corresponding  prolongations  of  the  plates  of  the  opposite  side,  so 
as  to  enclose  an  area  of  mesenchyme  into  which  the  chondrification 
only  extends  at  a  later  period,  and  occasionally  fails  to  so  extend, 
producing  what  is  termed  a  foramen  thyreoideum. 

The  mesenchymal  condensations  which  represent  the  cricoid 
and  arytenoid  cartilages  are  continuous,  but  each  arytenoid  has  a 
distinct  center  of  chondrification,  while  the  cartilage  of  the  cricoid 
appears  as  a  single  ring  which  is  at  first  open  dorsally  and  only  later 
becomes  complete.  The  epiglottis  cartilage  resembles  the  thyreoid 
in  being  formed  by  the  fusion  of  two  originally  distinct  cartilages, 
from  each  of  which  a  portion  separates  to  form  the  cuneiform 
cartilages  {cartilages  of  Wrisberg)  which  produce  the  tubercles  of 
the  same  name  on  the  ary-epiglottic  fold,  while  the  corniculate 
cartilages  (cartilages  of  Santorini)  are  formed  by  the  separation  of  a 
small  portion  of  cartilage  from  each  arytenoid. 

The  formation  of  the  thyreoid  cartilage  by  the  fusion  of  two  pairs 
of  lateral  elements  finds  an  explanation  from  the  study  of  the 
comparative  anatomy  of  the  larynx.  In  the  lowest  group  of  the 
mammalia,  the  Monotremata,  the  four  cartilages  do  not  fuse 
together  and  are  very  evidently  serially  homologous  with  the  car- 
tilages which  form  the  cornua  of  the  hyoid.  In  other  words,  the 
thyreoid  results  from  the  fusion  of  the  fourth  and  fifth  branchial 
cartilages.  The  cricoid,  in  its  development,  presents  such  striking 
similarities  to  the  cartilaginous  rings  of  the  trachea  that  it  is  probably 
to  be  regarded  as  the  uppermost  cartilage  of  that  series,  but  the 
epiglottis  seems  to  be  a  secondary  chondrification  in  the  glosso- 
laryngeal  fold  (Schaffer).  The  arytenoids  possibly  represent  an 
additional  pair  of  branchial  cartilages,  such  as  occur  in  the  lower 
vertebrates  (Gegenbaur). 

These  last  arches  have  undergone  almost  complete  reduction  in 
the  mammalia,  the  cartilages  being  their  only  representatives,  but, 


LITERATURE  337 

in  addition  to  the  cartilages,  the  fourth  and  fifth  arches  have  also 
preserved  a  portion  of  their  musculature,  part  of  which  becomes 
transformed  into  the  muscles  of  the  larynx.  Since  the  nerve  which 
corresponds  to  these  arches  is  the  vagus,  the  supply  of  the  larynx  is 
derived  from  that  nerve,  the  superior  laryngeal  nerve  probably 
corresponding  to  the  fourth  arch,  while  the  inferior  (recurrent) 
answers  to  the  fifth. 

The  course  of  the  recurrent  nerve  finds  its  explanation  in  the  relation 
of  the  nerve  to  the  fourth  branchial  artery.  When  the  heart  occupies 
its  primary  position  ventral  to  the  floor  of  the  pharynx,  the  inferior 
laryngeal  nerve  passes  transversely  inward  to  the  larynx  beneath  the 
fourth  branchial  artery.  As  the  heart  recedes  the  nerve  is  caught  by  the 
vessel  and  is  carried  back  with  it,  the  portion  of  the  vagus  between  it  and 
the  superior  laryngeal  nerve  elongating  until  the  origins  of  the  two 
laryngeal  nerves  are  separated  by  the  entire  length  of  the  neck.  Hence  it 
is  that  the  right  recurrent  nerve  bends  upward  behind  the  right  subclav- 
ian artery,  while  the  left  curves  beneath  the  arch  of  the  aorta  (see 
Fig.  149). 

LITERATURE. 

J.  M.  Flint:  "The  Development  of  the  Lungs,"  Amer.  Journ.  Anal.,  vi,  1906. 

J.  E.  Frazer:  "The  Development  of  the  Larynx,"  Journ.  Anat.  and Phys.,  xliv,  1910. 

E.  Goppert:  "Ueber  die  Herkunft  der  Wrisbergschen  Knorpels,"  Morphol.  Jahrbuch, 

xxi,  1894. 
W.  His:  "Zur  Bildungsgeschichte  des  Lungen  beim  menschlichen  Embryo,"  Archiv 

fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1887. 
E.  Kallius:  "Beitrage  zur  Entwickelungsgeschichte  des  Kehlkopfes,"  Anat.  Hefle, 

ix,  1897. 
E.  Kallius:  "Die  Entwickelung  des  menschlichen  Kehlkopfes,"  Verhandl.  der  Anat. 

Gesellsch.,  xii,  1898. 
A.  Lisser:   "Studies  on  the  Development  of  the  Human  Larynx,"   Amer.  Journ. 

Anat.,  xii,  191 1. 
A.  Narath:  "Der  Bronchialbaum  der  Saugethiere  und  des  Menschen,"  Bibliotheca 

Medica,  Abth.  A,  Heft  3,  1901. 
J.   Schaffer:   "Zur  Histologie   Histogenese   und   phylogenetischen   Bedeutung   der 

Epiglottis,"  Anat.  Hefte,  xxxin,  1907. 
A.   Sotjlie!  and   E.   Bardier:   "Recherches  sur  le  developpement  du  larynx  chez 

l'homme,"  Journ.  de  V Anat.  et  de  la  Physiol.,  xxiii,  1907. 


CHAPTER  XIII. 

THE   DEVELOPMENT  OF  THE  URINOGENITAL  SYSTEM. 

The  excretory  and  reproductive  systems  of  organs  are  so  closely 
related  in^their  development  that  they  must  be  considered  together. 
They  both  owe  their  origin  to  the  mesoderm  which  constitutes,  the 
intermediate  cell-mass  (p.  77),  this,  at  an  early  period  of  develop- 
ment, becoming  thickened  so  as  to  form  a  ridge  projecting  into  the 
dorsal  portion  of  the  ccelom  and  forming  what  is  known  as  the 
Wolffian  ridge  (Fig.  207,  wr).     The  greater  portion  of  the  substance 


,nc 


6    y L         m         y7 


otrwr 


Fig.  207. — Transverse  Section  through  the  Abdominal  Region  of  a  Rabbit 

Embryo  of  12  mm. 

a,  Aorta;  gl.,  glomerulus;  gr,  genital  ridge;  m,  mesentery;  nc,  notochord;  t,  tubule  of 

mesonephros;  wd,  Wolffian  duct;  wr,  Wolffian  ridge. — (Mihalkovicz.) 

of  this  ridge  is  concerned  in  the  development  of  the  primary  and 
secondary  excretory  organs,  but  on  its  mesial  surface  a  second  ridge 
appears  which  is  destined  to  give  rise  to  the  ovary  or  testis,  and 
hence  is  termed  the  genital  ridge  (gr). 

The  development  of  the  excretory  organs  is  remarkable  in  that 
three  sets  of  organs  appear  in  succession.  The  first  of  these,  the 
pronephros,  exists  only  in  a  rudimentary  condition  in  the  human 

338 


THE    PRONEPHROS 


339 


embryo,  although  its  duct,  the  pronephric  or  Wolffian  duct,  undergoes 
complete  development  and  plays  an  important  part  in  the  develop- 
ment of  the  succeeding  organs  of  excretion  and  also  in  that  of  the 
reproductive  organs.  The  second  set,  the  mesonephros  or  Wolffian 
body,  reaches  a  considerable  development  during  embryonic  life, 
but  later,  on  the  development  of  the  final  set,  the  definite  kidney  or 
metanephros,  undergoes  degeneration,  portions  only  persisting  as 
rudimentary  structures  associated  for  the  most  part  with  the  repro- 
ductive organs. 

The  Development  of  the  Pronephros  and  the  Pronephric 

Duct. — The  first  portions  of  thppjrrejl?ry  system  to  make  their 
appearance  are  the  pronephric  orWolffian  ducts,  which  develop  as 


1/71 


nc 


en 


Fig.^2o8. — Transverse  Section  through  Chick  Embryo  of  about  Thirty-six 

Hours. 

en,  Endoderm;  im,  intermediate  cell  mass;  ms,  mesodermic  somite;  nc,  notochord;  so, 

somatic,  and  sp,  splanchnic  mesoderm;  wd,  Wolffian  duct. — (Waldeyer.) 

outgrowths  of  the  dorsal  wallsof_the  intermediatecejljnasses;  At  first 
ThT outgrowths  are  solid  cords  of  cells  (Fig.  208,  wd),  but" later  a 
lumen  appears  in  the  center  of  each  and  the  canal  so  formed  from 
each  intermediate  cell  mass,  bending  backward  at  its  free  end,  comes 
into  contact  and  fuses  with  the  canal  from  the  next  succeeding 
segment.  Two  longitudinal  canals,  the  pronephric  or  Wolffian  ducts, 
are  thus  formed,  with  which  the  cavities  of  the  intermediate  cell  masses 
communicate.  The  formation  of  the  ducts  begins  in  the  anterior 
segments  before  the  segmentation  of  the  posterior  portions  of  the 
mesoderm  has  taken  place,  and  the  further  backward  extension  of 
the  ducts  takes  place  independently  of  the  formation  of  excretory 
tubules,  apparently  by  a  process  of  terminal  growth.     The  free  end 


340  THE   PRONEPHROS 

of  each  duct  comes  into  intimate  relation  with  the  ectoderm  above  it, 
so  much  so  that  its  posterior  portion  has  been  held^by  some  observers 
to  be  formed  from  that  layer,  but  it  seems  more  probable  that  the 
relation  to  the  ectoderm  is  a  secondary  process  and  that  the  ducts 
are  entirely  of  mesodermal  origin.  They  reach  the  cloaca  in  em- 
bryos of  a  little  over  4  mm.,  and  later  they  unite  with  that  organ,  so 
that  their  lumina  open  into  its  cavity. 

The  pronephric  tubules  make  their  appearance  in  embryos  of 
about  1.7  mm.,  while  as  yet  there  are  only  nine  or  ten  mesodermic 
somites,  and  they  are  formed  from  the  intermediate  cell  masses  of  the 
seventh  to  the  fourteenth  segment,  and  perhaps  from  those  situated 


En      Ao 


Fig.  209. — Diagram  showing  the  Structure  of  a  Fully  Developed 
Pronephric  tubule. 
Ao,  Aorta;  Coe,  ccelom;  ec,  Ectoderm;  eg,  external  glomerulus;  en,  endoderm;  Ms, 
mesodermic  somite;  iV,  nervous  system;  n,  nephrostome;  nc,  notochord;  pc,  pronephric 
chamber;  Wd,  Wolffian  duct. — (Modified  from  Felix.) 

still  more  anteriorly.  The  entire  series,  however,  is  never  in  exist- 
ence at  any  one  time,  for  before  the  more  posterior  tubules  are 
formed,  those  of  the  anterior  segments  have  undergone  degeneration. 
Each  pronephric  tubule,  when  fully  formed,  consists  of  a  portion 
which  unites  it  to  the  Wolffian  duct,  and  opens  at  its  other  end  into 
an  enlargement,  the  pronephric  chamber,  (Fig.  209,  pc),  which,  on 
its  part  opens  mto  the  ccelomic  cavity  by  means  of  a  nephrostome 
canal.  In  the  neighborhood  of  the  ccelomic  opening,  or  nephrostome, 
an  outgrowth  of  the  ccelomic  epithelium  is  formed,  and  a  branch 
from  the  aorta  penetrates  into  this  to  form  a  stalked  external  glomer- 
ulus lying  free  in  the  coelomic  cavity    (Fig.    209,   e.g.).     Internal 


THE  MESONEPHROS 


341 


glomeruli,  such  as  occur  in  connection  with  the  mesonephric  tubules 
do  not  occur  in  the  pronephros  of  the  human  embryo,  and  this  fact, 
together  with  the  presence  of  external  glomeruli  and  the  participa- 
tion of  the  tubules  in  the  formation  of  the  Wolffian  duct,  serve  to 
distinguish  the  pronephros  from  the  mesonephros. 

The  pronephric  tubules,  are,  as  has  been  stated,  transitory 
structures  and  by  the  time  the  embryo  has  reached  a  length  of  about 
5  mm.  they  have  all  disappeared.  Before  their  disappearance  is 
complete,  however,  a  second  series  of  tubules  has  commenced  to 
develop,  forming  what  is  termed  the  mesonephros  or  Wolffian  body. 

The  Development  of  the  Mesonephros. — The  pronephric 
duct  does  not  disappear  with  the  degeneration  of  the  pronephric 
tubules,  but  persists  to  serve  as 
the  duct  for  the  mesonephros  and 
to  play  an  important  part  in  the 
development  of  the  metanephros 
also.  In  the  Wolffian  ridge  there 
appear  in  embryos  of  between  3 
and  4  mm.  a  number  of  coiled 
tubules,  which  arise  by  some  of 
the  cells  of  the  ridge  aggregating 
to  form  solid  cords,  at  first  en- 
tirely unconnected  with  either  the 
coelomic  epithelium  or  the 
Wolffian  duct.  Later  the  cords 
become  connected  with  the  cce- 
lomic  epithelium  and  acquire  a 
lumen,  and  near  the  coelomic  end 

of  the  tubule,  at  a  region  corresponding  to  the  chamber  of  a  pro- 
nephric tubule,  a  condensation  of  the  mesenchyme  of  the  Wolffian 
ridge  occurs  to  form  a  glomerulus  into  which  a  branch  extends  from 
the  neighboring  aorta.  The  tubules  finally  acquire  connection  with 
the  Wolffian  duct  and  at  the  same  time  lose  their  connections  with 
the  coelomic  epithelium,  their  nephrostomes  being  accordingly  but 
transitory  structures.     The  tubules  rapidly  increase  in  length  and 


Fig.  210. — Transverse  Section  of 
the  Wolffian  Ridge  of  a  Chick  Em- 
bryo of  Three  Days. 

ao,  Aorta;  gl,  glomerulus;  gr,  genital 
ridge;  mes,  mesentery;  ml,  mesonephric 
tubule;  vc,  cardinal  vein;  Wd,  Wolffian 
duct. — (Mihalkovicz.) 


342 


THE    MESONEPHROS 


become  coiled,  and  the  glomeruli  project  into  their  cavities,  pushing 
in  front  of  them  the  wall  of  the  tubule  so  that  it  has  the  appearance 
represented  in  Fig.  210. 

In  its  anterior  portion  the  Wolffian  ridge  is  formed  by  distinct 
intermediate  cell  masses,  but  posterior  to  the  tenth  segment  it 
becomes  distinguishable  from  the  rest  of  the  mesoderm  before  this 
has  become  segmented,  and,  failing  to  undergo  transverse  division 
into  segments,  it  forms  a  continuous  column  of  cells,  known  as  the 
nephrogenic  cord.  The  anterior  tubules  of  the  mesonephros  make 
their  appearance  in  the  intermediate  cell  masses  belonging  to  the 
sixth  cervical  segment,  its  tubules  thus  overlapping  those  of  the 
pronephros,  and  from  this  level  they  appear  in  all  succeeding  seg- 
ments and  in  the  nephrogenic  cord  as  far  back  as  the  region  of  the 
third  or  fourth  lumbar  segment,  where  the  cord  is  partially  inter- 
rupted. This  interruption  marks  the  dividing  line  between  the  meso- 
nephric  and  metanephric  portions  of  the  cord,  the  portions  posterior 
to  it  being  destined  to  give  rise  to  the  metanephros.  But,  as  is  the 
case  with  the  pronephros,  the  entire  series  of  mesonephric  tubules  is 
never  in  existence  at  any  one  time,  a  degeneration  of  the  anterior 
ones  supervening  even  before  the  posterior  ones  have  differentiated, 
and  the  degeneration  proceeds  to  such  an  extent  that  in  an  embryo 
of  about  21  mm.  all  the  tubules  of  the  cervical  and  thoracic  segments 
have  disappeared,  only  those  of  the  lumbar  segments  persisting. 

This  does  not  mean,  however,  that  the  number  of  persisting 
tubules  corresponds  with  that  of  the  segments  in  which  they  occur,  for 
the  tubules  are  not  segmental  in  their  arrangement,  but  are  much 
more  numerous  than  such  an  arrangement  would  allow.  Two, 
three,  or  even  as  many  as  nine  may  correspond  with  the  extent  of 
a  mesodermic  somite  and  when  the  reduction  is  complete  in  an  embryo 
of  21  mm.,  where  only  the  tubules  corresponding  with  four  or  five 
segments  remain,  they  may  number  twenty-six  in  each  mesonephros 
(Felix).  This  arrangement  of  the  tubules  together  with  the  size 
which  they  assume  when  fully  developed  brings  it  about  that  the 
Wolffian  ridges  become  somewhat  voluminous  structures  in  their 
mesonephric  portions,  projecting  markedly  into  the  ccelomic  cavity 


THE    METANEPHROS 


343 


(Fig.  211).  Each  is  attached  to  the  dorsal  wall  of  the  body  by  a  dis- 
tinct mesentery  and  has  in  its  lateral  portion,  embedded  in  its 
substance,  the  Wolffian  duct,  while  on  its  mesial  surface  anteriorly 
is  the  but  slightly  developed  genital  ridge  (/).  This  condition  is 
reached  in  the  human  embryo  at  about  the  sixth  or  seventh  week  of 
development,  and  after  that  period  the  mesonephros  again  begins  to 
undergo  rapid  degeneration,  so  that  at  about  the  sixteenth  week 


Fig.  211. — Urinogenital  Apparatus  of  a  Male  Pig  Embryo  of  6  cm. 

ao,  Aorta;  b,  bladder;  gh,  gubernaculum  testis;  k,  kidney;  md,  Mullerian  duct;  sr, 

suprarenal  body;  t,  testis;  w,  Wolffian  body;  wd,  Wolffian  duct. — (Mihalkovicz.) 


nothing  remains  of  it  except  the  duct  and  a  few  small  rudiments 
whose  history  will  be  given  later. 

The  Development  of  the  Metanephros. — The  first  indication 
of  the  metanephros  or  permanent  kidney  is  a  tubular  outgrowth 
from  the  dorsal  surface  of  the  Wolffian  duct  shortly  before  its 
entrance  into  the  cloaca  (Fig.  170).  When  first  formed  this  out- 
growth lies  lateral  to  the  posterior  portion  of  the  Wolffian  ridge, 


344 


THE    METANEPHROS 


which,  as  has  already  been  noted  (p.  342),  is  separated  from  the 
portion  that  gives  rise  to  the  mesonephros.  This  terminal  portion  of 
the  ridge  forms  what  is  termed  the  metanephric  blastema  and  in 
embryos  of  7  mm.  it  has  come  into  relation  with  the  outgrowth  from 
the  Wolffian  duct  and  covers  its  free  extremity  as  a  cap.  Since 
both  the  blastema  and  the  outgrowth  from  the  Wolffian  duct  take 

part  in  the  formation  of  the 
uriniferous  tubules,  these  have 
a  double  origin. 

The  outgrowth  from  the 
Wolffian  duct  as  it  continues  to 
elongate  comes  to  lie  dorsal  to 
the  mesonephros,  carrying  the 
cap  of  blastema  with  it,  and 
it  soon  assumes  a  somewhat 
club-shaped  form,  its  terminal 
enlargement  or  ampulla  form- 
ing what  may  be  termed  the 
primary  renal  pelvis,  while  the 
remainder  represents  theureter. 
The  primary  renal  pelvis  then  gives  rise  to  from  three  to  six,  usually 
four,  tubular  outgrowths,  which  may  be  termed  primary  collecting 
tubules,  and  with  their  formation  the  original  cap  of  metanephric 
blastema  undergoes  a  division  into  as  many  portions  as  there  are 
tubules,  so  that  each  of  the  latter  has  its  own  cap  of  blastema. 
As  soon  as  each  tubule  has  reached  a  certain  length  it  begins  to 
enlarge  at  its  free  extremity  to  form  an  ampulla,  just  as  did  the 
primary  renal  pelvis,  and  from  this  ampulla  there  grow  out  from 
two  to  four  secondary  collecting  tubules,  a  further  corresponding 
division  of  the  metanephric  blastema  taking  place.  In  their  turn 
these  secondary  tubules  similarly  enlarge  at  their  extremities  to 
form  ampullae  (Fig.  212,  A)  from  which  tertiary  collecting  tubules 
are  budded  out,  accompanied  by  a  third  fragmentation  of  the  blastema 
and  so  the  process  goes  on  until  about  the  fifth  fetal  month,  the 
number  of  generations  of  collecting  tubules  formed  being  between 


Fig.  212. — Diagrams  of  Early  Stages  in 
the  Development  of  the  Metanephric 
Tubules. 

t,  Urinary  tubule;  Ur,  ureter;  v,  renal  am- 
pulla.— (Haycrqft.) 


THE    METANEPHROS 


34- 


eleven  and  thirteen,  each  tubule  of  the  final  generation  having  its 
cap  of  blastema. 

In  this  way  there  is  formed  a  complicated  branching  system  of 
tubules  all  of  which  ultimately  communicate  with  the  primary 
renal  pelvis,  and  all  of  which  have,  in  the  last  analysis,  had  their 
origin  from  the  Wolffian  duct.  They  represent,  however,  only  the 
collecting  portions  of  the  uriniferous  tubules,  their  excreting  por- 


Fig.  213. — Four  Stages  in  the  Development  of  a  Uriniferous  Tubule  of  a  Cat. 
A,  Arched  collecting  tubule,  C,  distal  convoluted  tubule;  C,  proximal  convoluted 
tubule;  H,  loop  of  Henle;  M,  glomerulus;  T,  renal  vesicle;  V,  ampulla  (drawn  from 
reconstructions  prepared  by  G.  C.  Huber). 

tions  having  yet  to  form,  and  these  take  their  origin  from  the  meta- 
nephric  blastema. 

When  the  terminal  collecting  tubules  have  been  formed  the 
blastemic  cap  in  connection  with  each  one  condenses  to  form  a  renal 
vesicle  (Fig.  213,  A,  T),  which  is  at  first  solid,  but  later  becomes 
hollow  and  proceeds  to  elongate  to  an  S-shaped  tubule,  one  end  of 
which  becomes  continuous  with  the  neighboring  ampulla  (Figs. 
212,  B,  and  213,  B),  and  in  the  space  enclosed  by_what  may  be 
termed  the  lower  loop  of  the  S  a  collection  of  mesenchyme  cells 


346  THE    METANEPHROS 

appears,  into  which  branches  penetrate  at  an  early  stage  from  the 
renal  artery  to  form  a  glomerulus,  the  neighboring  walls  of  the 
tubule  becoming  exceedingly  thin  and  being  transformed  into  a 
capsule  of  Bowman.  The  upper  loop  of  the  S  now  begins  to  elon- 
gate (Fig.  213,  C),  growing  toward  the  hilus  of  the  kidney,  parallel 
to  the  branch  of  the  outgrowth  from  the  Wolffian  duct  to  which  it  is 
attached  and  between  this  and  the  glomerulus,  and  forms  a  loop  of 
Henle.  From  the  portion  of  the  horizontal  limb  of  the  S  which  lies 
between  the  glomerulus  and  the  descending  limb  of  the  loop  of 
Henle  the  proximal  convoluted  tubule  (C)  arises,  while  the  distal 
convoluted  and  the  arched  collecting  tubules  (C  and  A)  are  formed 
from  the  uppermost  portion  of  the  upper  loop  (Fig.  213,  D).  The 
entire  length  of  each  uriniferous  tubule  from  Bowman's  capsule  to 
the  arched  collecting  tubule  inclusive  is  thus  derived  from  a  renal 
vesicle,  that  is  to  say,  from  the  metanephric  blastema. 

Since  the  tubules  of  the  kidney  are  formed  by  the  union  of  two  originally 
distinct  structures  it  is  conceivable  that  in  the  cases  of  certain  tubules 
there  may  be  a  failure  of  the  union.  The  blastemic  portion  of  the  tubules 
would,  nevertheless,  continue  their  development  and  become  functional 
and,  since  there  would  be  not  means  of  escape  for  the  secretion,  the  result 
would  be  a  cystic  kidney.  Occasionally  the  two  blastemata  of  opposite 
sides  fuse  across  the  middle  line,  the  result  being  the  formation  of  a 
single  transverse  or  horse-shoe  shaped  kidney,  or,  what  is  much  rarer,  the 
blastema  of  one  side  may  cross  the  middle  line  to  fuse  with  that  of  the 
other,  the  result  being  an  apparently  single  kidney  with  two  ureters  which 
open  normally  into  the  bladder. 

The  primary  renal  pelvis  is  the  first  formed  ampulla  and  does  not 
exactly  represent  the  definitive  pelvis.  This  is  produced  partly  by 
the  enlargement  of  the  primary  pelvis  and  partly  by  the  enlargement 
of  the  collecting  tubules  of  the  first  four  generations,  those  of  the  third 
and  fourth  generations  later  being  taken  up  or  absorbed  into  those 
of  the  second  generation,  so  that  the  tubules  of  the  fifth  generation 
appear  to  open  directly  into  those  of  the  second,  which  form  the 
calices  minores,  while  those  of  the  first  constitute  the  calices  majores. 
In  some  kidneys  the  process  of  reduction  of  the  earlier  formed 
collecting  tubules  proceeds  a  step  further,  those  of  the  first  generation 


THE    MULLERIAN    DUCT  347 

being  taken  up  into  the  primary  renal  pelvis,  the  secondaries  then 
forming  a  series  of  short  calices  arising  from  a  single  pelvic  cavity. 

At  about  the  tenth  week  of  development  the  surface  of  the  human 
kidney  becomes  marked  by  shallow  depressions  into  lobes,  of  which 
there  are  about  eighteen,  one  corresponding  to  each  of  the  groups 
of  tubules  which  arise  from  the  same  renal  vesicle.  This  lobation 
persists  until  after  birth  and  then  disappears  completely,  the  surface 
of  the  kidney  becoming  smooth. 

The  Development  of  the  Mullerian  Duct  and  of  the  Genital 
Ridge. — At  the  time  when  the  Wolffian  body  has  almost  reached 
its  greatest  development  the  Wolffian  ridge  is  distinctly  divided  into 
three  portions  (Fig.  214),  a  median  or  mesonephric  portion  attached 
to  the  body  wall,  a  lateral  or  tubal  portion  containing  the  Wolffian 
duct  and  attached  to  the  mesonephric  portion,  and  a  genital  portion, 
formed  by  the  genital  ridge  and  also  attached  to  the  mesonephric 
portion,  but  to  its  medial  surface.  In  the  tubal  portion  a  second 
longitudinal  duct,  known  as  the  Mullerian  duct  (Fig.  214,  Md), 
makes  its  appearance.  Near  the  anterior  end  of  each  Wolffian 
ridge  there  is  formed  on  the  free  edge  of  the  tubal  portion  an  invag- 
ination of  the  peritoneal  covering,  and  by  the  proliferation  of  the 
cells  at  its  tip  this  invagination  gradually  extends  backward  in  the 
substance  of  the  tubal  portion  and  reaches  the  cloaca  in  embryos  of 
about  22  mm.  The  primary  peritoneal  invagination  becomes  the 
abdominal  ostium  of  the  Mullerian  duct,  the  backward  prolongation 
forming  the  duct  itself. 

In  Fig.  214  it  will  be  seen  that  the  tubal  portion  of  the  left 
Wolffian  ridge  is  somewhat  bent  inward  toward  the  median  line 
and  in  the  lower  parts  of  their  extent  this  becomes  more  pronounced 
in  both  tubal  portions  until  finally  their  free  edges  come  in  contact 
and  fuse  in  the  median  line,  while  at  the  same  time  their  lower  edges 
fuse  with  the  floor  of  the  ccelomic  cavity.  In  this  way  a  transverse 
partition  is  formed  across  what  will  eventually  be  the  pelvis  of  the 
adult,  this  cavity  being  thus  divided  into  two  compartments,  a 
posterior  one  containing  the  lower  portion  of  the  intestine  and  an  ante- 
rior one  containing  the  bladder.     With  the  formation  of  this  trans- 


348 


THE    GENITAL   RIDGE 


N 


M 


■sg 


M 


M 


S 


Vs> 


Ao 


V 


-r 


M 


Ur 


Wd 


Md 
M 


(B 


I 

! 

UA 


RA 


Fig.  214. — Transverse  Section  through  the  Abdominal  Region  oe  an  Embryo 

of  25  MM. 
_  Ao,  Aorta;  B,  bladder;  I,  intestine;  L,  liver;  M,  muscle;  Md,  Miillerian  duct;  N, 
spinal  cord;    Ov,  ovary;  RA,  rectus  abdominis;  Sg,   spinal  ganglion;    UA,   umbilical 
artery;  Ur,  ureter;  V,  vertebra;  W,  Wolffian  body;  Wd,  Wolffian  duct. — (Keibel.) 


THE    GENITAL   RIDGE  349 

verse  fold,  which  is  represented  by  the  broad  ligament  in  the  female, 
the  Miillerian  ducts  of  opposite  sides  are  brought  into  contact  and 
finally  fuse  in  the  lower  portions  of  their  course  to  form  an  unpaired 
utero-vaginal  canal. 

Upon  the  lateral  surface  of  the  mesonephric  portion  of  the 
Wolffian  ridge  a  longitudinal  elevation  is  formed  at  about  this  time. 
It  is  the  inguinal  fold  and  on  the  union  of  the  transverse  fold  with 
the  floor  of  the  ccelomic  cavity  it  comes  into  contact  and  fuses  with  the 
lower  part  of  the  anterior  abdominal  wall,  just  lateral  to  the  lateral 
border  of  the  rectus  abdominis  muscle.  In  the  substance  of  the 
fold  the  mesenchyme  condenses  to  form  a  ligament-like  cord,  the 
inguinal  ligament,  whose  further  history  will  be  considered  later  on. 

The  genital  ridge  makes  its  apearance  as  a  band-like  thickening 
of  the  epithelium  covering  the  mesial  surface  of  the  Wolffian  ridge 
(Fig.  207,  gr).  Later  columns  of  cells  grow  down  from  the  thicken- 
ing into  the  substance  of  the  Wolffian  ridge,  displacing  the  mesoneph- 
ric tubules  to  a  greater  or  less  extent.  These  columns  are  com- 
posed of  two  kinds  of  cells:  (i)  smaller  epithelial  cells  with  a  rela- 
tively small  amount  of  cytoplasm  and  (2)  large,  spherical  cells  with 
more  abundant  and  clear  cytoplasm  known  as  sex-cells.  The 
growth  of  the  cell-columns  down  into  the  substance  of  the  Wolffian 
body  does  not  take  place,  however,  to  an  equal  extent  in  all  por- 
tions of  the  length  of  the  genital  ridge.  Indeed,  three  regions 
may  be  recognized  in  the  ridge;  an  anterior  one  in  which  a  relatively 
small  number  of  cell-columns,  extending  deeply  into  the  stroma,  is 
formed;  a  middle  one  in  which  numerous  columns  are  formed;  and 
a  posterior  one  in  which  practically  none  are  formed.  The  first 
region  has  been  termed  the  rete  region  and  its  cell-columns  the  rete- 
cords,  the  second  region  the  sex-gland  region  and  its  columns  the 
sex-cords,  and  the  posterior  region  is  the  mesenteric  region  and  plays 
no  part  in  the  actual  formation  of  the  ovary  or  testis. 

In  the  human  embryo  all  the  sex-cells  seem  to  have  their  origin  from 
the  epithelium  of  the  genital  ridge,  but  in  the  lower  vertebrates  and  also 
in  mammals  (Allen,  Rubaschkin)  they  have  been  found  to  make  their 
appearance  in  the  endoderm  of  the  digestive  tract.     Thence  they  wander 


35° 


THE    TESTIS 


into  the  mesentery  and  some  of  them  eventually  into  the  peritoneum 
covering  the  mesial  surface  of  the  Wolffian  ridge,  where  they  give  rise 
to  the  sex-cells  found  in  the  epithelium  of  the  genital  ridge.  This  origin 
of  the  sex-cells  has  not  yet  been  observed  in  the  human  embryo. 

The  various  steps  in  the  differentiation  of  the  reproductive 
organs  so  far  described  occur  in  all  embryos,  no  matter  what  their 
future  sex  may  be.  The  later  stages,  however,  differ  according  to 
sex,  and  consequently  it  will  be  necessary  to  follow  the  further 
development  first  of  the  testis  and  then  of  the  ovary,  the  changes 


Fig.  215. — Section  through  the  Testis  and  the  Broad  Ligament  of  the  Testis 
of  an  Embryo  of  5.5  mm. 

ep,  Epithelium;  md,  Miillerian  duct;  mo,  mesorchium;  re,  rete-cords;  sc,  sex-cords;  wd, 
Wolffian  duct. — (Mihalkovicz.) 

that  take  place  in  the  ducts  and  other  accessory  structures  being 
reserved  for  a  special  section. 

The  Development  of  the  Testis. — At  about  the  fourth  or  fifth  week 
there  appears  in  the  sex-gland  region  of  the  genital  ridge  a  structure 
which  serves  to  characterize  the  region  as  a  testis.  This  is  a  layer 
of  somewhat  dense  connective  tissue  which  grows  in  between  the 
epithelial  and  stroma  layers  of  the  sex-gland  region  and  gradually 
extends  around  almost  the  entire  sex-gland  to  form  the  tunica  albu- 
ginea.     By  its  development  the  sex-cords  are  separated  from  the 


THE    TESTIS 


351 


epithelium,  which  later  becomes  much  flattened  and  eventually 
almost  disappears.  Shortly  after  the  appearance  of  the  albuginea 
the  sex-cords  unite  to  from  a  complicated  network  and  the  rete-cords 
grow  backward  along  the  line  of  attachment  of  the  testis  to  the 
mesonephric  portion  of  the  Wolffian  ridge,  coming  to  lie  in  the  hilus 


Mc   — 


ep 


■-  R 


—  Mn 


Fig.  216. — Longitudinal  Section  of  the  Ovary  of  an  Embryo  Cat  of  9.4  cm. 
cor,  Cortical  layer;  ep,  epoophoron;  Mc,  medullary  cords;  Mn,  mesonephros;  pf, 
peritoneal  fold  containing  Fallopian  tube;  R,  rete;  T,  Fallopian  tube. — {Coert,  from 
Bilhler.) 

of  the  testis  (Fig.  215).  They  then  develop  a  lumen  and  send  off 
branches  which  connect  with  the  sex-cord  reticulum  and  they  also 
make  connection  with  the  glomerular  portions  of  the  tubules  belong- 
ing to  the  anterior  part  of  the  mesonephros.  Since  like  the  sex- 
cords,  they  have  by  this  time  separated  from  the  epithelium  that 


352  THE    OVARY 

gave  rise  to  them,  they  now  extend  between  the  sex-cord  reticulum 
and  the  anterior  mesonephric  tubules.  Certain  portions  of  the 
sex-cords  now  begin  to  break  down  leaving  other  portions  to  form 
convoluted  stems  which  eventually  become  the  seminiferous  tubules, 
while  from  the  rete-cords  are  formed  the  tubuli  recti  and  rete  testis, 
by  which  the  spermatozoa  are  transmitted  to  the  mesonephric 
tubules  and  so  to  the  Wolffian  duct  (see  p.  355). 

The  development  of  the  seminiferous  tubules  is  not,  however, 
completed  until  puberty.  The  stems  derived  from  the  sex-cords 
form  cylindrical  cords,  between  which  lie  stroma  cells  and  in- 
terstitial cells  derived  from  the  stroma;  but  until  puberty  these  cords 
remain  solid,  a  lumen  developing  only  at  that  period.  The  cords 
contain  the  same  forms  of  cells  as  were  described  as  occurring  in  the 
epithelium  of  the  germinal  ridge,  and  while  in  the  early  stages 
transitional  forms  seem  to  occur,  in  later  periods  the  two  varieties  of 
cells  are  quite  distinct,  the  sex-cells  becoming  spermatogonia 
(see  p.  14)  and  being  the  mother  cells  of  the  spermatozoa,  while  the 
remaining  epithelial  cells  perhaps  become  transformed  into  the  con- 
nective-tissue walls  of  the  tubules. 

The  Development  of  the  Ovary. — In  the  case  of  the  ovary,  after 
the  formation  of  the  sex-cords,  connective  tissue  grows  in  between 
these  and  the  epithelium,  forming  a  layer  equivalent  to  the  tunica 
albuginea  of  the  testis.  It  is,  however,  a  much  looser  tissue  than 
its  homologue  in  the  male,  and,  indeed,  does  not  completely  isolate 
the  sex-cords  from  the  epithelium,  although  the  majority  of  the  cords 
are  separated  and  sink  into  the  deeper  portions  of  the  ovary  where 
they  form  what  have  been  termed  the  medullary  cords.  In  the  mean- 
time the  germinal  epithelium  has  continued  to  bud  off  cords  which 
unite  to  form  a  cortical  layer  of  cells  lying  below  the  epithelium  and 
separated  from  the  medullary  cords  by  the  tunica  albuginea 
(Fig.  216). 

Later  the  cortical  layer  becomes  broken  up  by  the  ingrowth  of 
stroma  tissue  into  spherical  or  cord-like  masses,  consisting  of  sex- 
cells  and  epithelial  cells  (Fig.  217).  The  invasion  of  the  stroma 
continuing,  these  spheres  or  cords  (Pfluger's  cords)  become  divided 


THE    OVARY 


353 


into  smaller  masses,  the  primary  ovarian  follicles,  each  of  which 
consists  as  a  rule  of  a  single  sex-cell  surrounded  by  a  number  of 
epithelial  cells,  the  whole  being  enclosed  by  a-  zone  of  condensed 
stroma  tissue,  which  eventually  becomes  richly  vascularized  and 
forms  a  theca  folliculi  (Fig.  10).  The  epithelial  cells  in  each  follicle 
are  at  first  comparatively  few  in  number  and  closely  surround  the 
sex-cell  (Fig.  217,/),  which  is  destined  to  become  an  ovum,  but  in 
certain  of  the  follicles  they  undergo  an  increase  by  mitosis,  becoming 
extremely  numerous,  and  later 
secrete  a  fluid,  the  liquor  folli- 
culi, which  collects  at  one  side  of 
the  follicle  and  eventually  forms 
a  considerable  portion  of  its  con- 
tents. The  follicular  cells  are 
differentiated  by  its  appearance 
into  the  stratum  granulosum, 
which  surrounds  the  wall  of  the 
follicle,  and  the  discus  froligerus, 
in  which  the  ovum  is  embedded 
(Fig.  10,  dp),  and  the  cells  which 
immediately  surround  the  ovum, 
becoming  cylindrical  in  shape, 
give  rise  to  the  corona  radiata 
(Fig.  11,  cr). 

A  somewhat  similar  fate  is 
shared  by  the  medullary  cords,  these  also  breaking  up  into  a  num- 
ber of  follicles,  but  sooner  or  later  these  follicles  undergo  degenera- 
tion so  that  shortly  after  birth  practically  no  traces  of  the  cords  re- 
main. It  must  be  noted  that  degeneration  of  the  follicles  formed 
from  the  cortical  layer  also  takes  place  even  during  fetal  life  and 
continues  to  occur  throughout  the  entire  periods  of  growth  and  func- 
tional activity,  numerous  atretic  follicles  being  found  in  the  ovary 
at  all  times.  Indeed  it  would  seem  that  degeneration  is  the  fate  of 
the  great  majority  of  the  follicles  and  sex-cells  of  the  ovary,  but  few 
ova  coming  to  maturity  during  the  life-time  of  any  individual. 
23 


Fig.  217. — Section  of  the  Ovary  of 
a  New-born  Child. 

a,  Ovarial  epithelium;  b,  proximal  part 
of  a  Pfl  tiger's  cord;  c,  sex-cell  in  epithe- 
lium; d  and  e,  spherical  masses;  /,  pri- 
mary follicle;  g,  blood-vessel. — (From 
Gegenbaur,  after  Waldeyer.) 


354  THE    GENITAL  DUCTS 

Rete-cords  developed  from  the  rete  portion  of  the  germinal 
ridge  occur  in  connection  with  the  ovary  as  well  as  with  the.  testis 
and  form  a  rete  ovarii  (Fig.  216,  R).  They  do  not,  however,  extend 
so  deeply  into  the  ovary,  remaining  in  the  neighborhood  of  the 
mesovarium,  and  they  do  not  become  tubular,  but  resemble  closely 
the  medullary  cords  with  which  they  are  serially  homologous. 
They  separate  from  the  epithelium  and  make  connections  with  the 
glomeruli  of  the  anterior  portion  of  the  mesonephros,  on  the 
one  hand,  and  on  the  other  with  medullary  cords,  and  in  later 
stages  show  a  tendency  to  break  up  into  primary  follicles,  which 
early  degenerate  and  disappear  like  those  of  the  medullary  cords. 

The  Transformation  of  the  Mesonephros  and  the  Ducts. — 
At  one  period  of  development  there  are  present,  as  representatives 
of  the  urinogenital  apparatus,  the  Wolffian  body  (mesonephros) 
and  duct,  the  Miillerian  duct,  and  the  developing  ovary  or  testis. 
Such  a  condition  forms  an  indifferent  stage  from  which  the  develop- 
ment proceeds  in  one  of  two  directions  according  as  the  genital 
ridge  becomes  a  testis  or  an  ovary,  the  Wolffian  body  in  part  under- 
going degeneration  and  in  part  persisting  to  form  organs  which  for 
the  most  part  are  rudimentary,  while  in  the  female  the  Wolffian 
duct  also  degenerates  except  for  certain  rudiments  and  in  the  male 
the  Miillerian  duct  behaves  similarly. 

In  the  Male. — It  has  been  seen  that  the  Wolffian  body,  through 
the  rete  cords,  enters  into  very  intimate  relations  with  the  testis, 
and  it  may  be  regarded  as  divided  into  two  portions,  an  upper 
genital  and  a  lower  excretory.  In  the  male  the  genital  portion 
persists  in  its  entirety,  serving  as  the  efferent  ducts  of  the  testis, 
which,  beginning  in  the  spaces  of  the  rete  testis,  already  shown  to  be 
connected  with  the  capsules  of  Bowman,  open  into  the  upper  part  of 
the  Wolffian  duct  and  form  the  globus  major  of  the  epididymis. 
The  excretory  portion  undergoes  extensive  degeneration,  a  portion 
of  it  persisting  as  a  mass  of  coiled  tubules  ending  blindly  at  both 
ends,  situated  near  the  head  of  the  epididymis  and  known  as  the 
paradidymis  or  organ  of  Giraldes,  while  a  single  elongated  tubule, 
arising  from  the  portion  of  the  Wolffian  duct  which  forms  the 


THE    GENITAL    DUCTS  355 

globus  minor  of  the  epididymis,  represents  another  portion  of  it  and 
is  known  as  the  vas  aberrans. 

The  Wolffian  duct  is  retained  complete,  the  portion  of  it  nearest 
the  testis  becoming  greatly  elongated  and  thrown  into  numerous 
coils,  forming  the  body  and  globus  minor  of  the  epididymis,  while  the 
remainder  of  it  is  converted  into  the  vas  deferens  and  the  ductus 
ejaculatorius.  A  lateral  outpouching  of  the  wall  of  the  duct  to 
form  a  longitudinal  fold  appears  at  about  the  third  month  and 
gives  rise  to  the  vesicula  seminalis,  the  lateral  position  of  the  out- 
growth explaining  the  adult  position  of  the  vesiculse  lateral  to  the 
vasa  deferentia. 

With  the  Mullerian  ducts  the  case  is  very  different,  since  they 
disappear  completely  throughout  the  greater  part  of  their  course, 
only  their  upper  and  lower  ends  persisting,  the  former  giving  rise  to  a 
small  sac-like  body,  the  sessile  hydatid  of  Morgagni,  attached  to  the 
upper  end  of  each  testis  near  the  epididymis.  It  has  been  seen  (p.  349) 
that  the  lower  ends  of  the  Mullerian  ducts,  in  the  male  as  well  as  the 
female,  fuse  to  form  the  utero-vaginal  canal,  and  the  lower  portion 
of  this  also  persists  to  form  what  is  termed  the  uterus  masculinus, 
although  it  corresponds  to  the  vagina  of  the  female  rather  than  to  the 
uterus.  It  is  a  short  cylindrical  pouch  of  varying  length,  that  opens 
into  the  urethra  at  the  bottom  of  a  depression  known  as  the  utriculus 
prostaticus  {sinus  pocularis). 

The  transverse  pelvic  partition,  produced  by  the  union  of  the  two 
tubal  portions  of  the  Wolffian  body,  is  formed  in  the  male  embryo, 
but  at  an  early  stage  its  anterior  surface  fuses  with  the  posterior 
surface  of  the  bladder  and  consequently  there  is  in  the  male  no  pelvic 
compartment  equivalent  to  the  vesico-uterine  pouch  of  the  female. 
The  male  recto-vesical  pouch  is,  however,  the  homologue  of  the  recto- 
uterine pouch  of  the  female. 

The  formation  of  the  inguinal  ligament  on  the  surface  of  the 
mesonephros  has  been  described  on  p.  349.  On  the  degeneration  of 
the  mesonephros  the  layer  of  peritoneum  that  covered  it  persists  to 
form  a  mesorchium  extending  from  the  body  wall  to  the  hilus  of  the 
testis  and  the  inguinal  ligament  now  comes  to  have  its  origin  from 


356  THE    GENITAL   DUCTS 

the  lower  pole  of  that  organ,  whence  it  extends  to  the  anterior  ab- 
dominal wall.  Owing  to  the  rudimentary  nature  of  the  uterus 
masculinus  and  the  slight  development  of  its  walls  the  inguinal 
ligament  does  not  become  involved  with  it,  but  remains  independent 
and  forms  the  gubemaculum  testis  of  the  adult,  whose  adult  posi- 
tion is  brought  about  by  the  descent  of  the  testis  into  the  scrotum 
(see  p.  366). 

In  the  Female. — In  the  female  the  transverse  partition  of  the 
pelvis  does  not  fuse  w'th  the  bladder  but  remains  distinct  as  the 
broad  ligament.  Consequently  there  is  in  the  female  both  a  vesico- 
uterine and  a  recto-uterine  pouch.  Since  the  genital  ridges  form 
upon  the  mesial  surfaces  of  the  Wolffian  ridges  and  the  tubal 
portions  are  their  lateral  portions,  when  these  latter  unite  to  form 
the  broad  ligament  the  ovary  will  come  to  lie  upon  the  posterior 
surface  of  that  structure,  projecting  into  the  recto-vesical  pouch. 
On  the  degeneration  of  the  mesonephros  the  peritoneum  that 
covered  it  becomes  a  part  of  the  broad  ligament,  forming  that  part 
of  it  which  contains  the  Fallopian  tubes  and  hence  is  known  as  the 
mesosalpinx,  while  the  lower  part  of  the  ligament,  on  account  of  its 
relation  to  the  uterus,. is  termed  the  mesometrium. 

The  genital  portion  of  the  mesonephros,  though  never  functional 
as  ducts  in  the  female,  persists  as  a  group  of  ten  to  fifteen  tubules, 
situated  between  the  two  layers  of  the  broad  ligament  and  in  close 
proximity  to  the  ovary;  these  constitute  what  is  known  as  the 
epoophoron  (parovarium  or  organ  of  Rosenmuller) .  The  tubules  ter- 
minate blindly  at  the  ends  nearest  the  ovary,  but  at  the  other  ex- 
tremity, where  they  are  somewhat  coiled,  they  open  into  a  collecting 
duct  which  represents  the  upper  end  of  the  Wolffian  duct.  Near  this 
rudimentary  body  is  another,  also  composed  of  tubules,  representing 
the  remains  of  the  excretory  portion  of  the  mesonephros  and  termed 
the  paroophoron  which,  however,  degenerates  during  the  early  years 
of  extra-uterine  life.  So  far  as  the  mesonephros  is  concerned,  there- 
fore, the  persisting  rudiments  in  the  female  are  comparable  to  those 
occurring  in  the  male. 

As  regards  the  ducts,  however,  the  case  is  different,  for  in  the 


THE    GENITAL    DUCTS  357 

female  it  is  the  Mtillerian  ducts  which  persist,  while  the  Wolmans 
undergo  degeneration,  a  small  portion  of  their  upper  ends  persisting 
in  connection  with  the  epoophora,  while  their  lower  ends  persist  as 
straight  tubules  lying  at  the  sides  of  the  vagina  and  forming  what 
are  known  as  the  canals  of  Gartner.  The  Mtillerian  ducts,  on  the 
other  hand,  become  converted  into  the  Fallopian  tubes  {tubas  uterince), 
and  in  their  lower  portions  into  the  uterus  and  vagina.  From  the 
margins  of  the  openings  by  which  the  Mullerian  ducts  communicate 
with  the  ccelom  projections  develop  at  an  early  period  and  give  rise 
to  the  fimbria,  with  the  exception  of  the  one  connected  with  the 
ovary,  the  fimbria  ovarica,  which  is  the  persisting  upper  portion  of 
the  original  genital  ridge.  From  the  utero-vaginal  canal  the  two 
structures  which  give  it  its  name  are  formed,  the  entire  canal  being 
transformed  into  the  mucous  membrane  of  the  uterus  and  vagina. 
Indeed,  the  lower  ends  of  the  Fallopian  tubes  are  also  taken  up 
into  the  uterus,  for  the  condensation  of  mesenchyme  that  takes 
place  around  the  mucosa  to  form  the  muscular  wall  of  the  uterus 
is  so  voluminous  that  it  includes  not  only  the  utero-vaginal  canal 
but  also  the  adjacent  portions  of  the  Mullerian  ducts.  The  histo- 
logical differentiation  of  the  uterus  from  the  vagina  begins  to 
manifest  itself  at  about  the  third  month,  and  during  the  fourth 
month  the  vaginal  portion  of  the  duct  becomes  flattened  and  the 
epithelium  lining  its  lumen  fuses  so  as  to  completely  occlude  it 
and,  a  little  later,  there  appears  at  its  lower  opening  a  distinct  semi- 
circular fold.  This  is  the  hymen,  a  structure  which  seems  to  be 
represented  in  the  male  by  the  colliculus  seminalis.  The  obliteration 
of  the  lumen  of  the  vagina  persists  until  about  the  sixth  month, 
when  the  cavity  is  re-established  by  the  breaking  down  of  the  central 
epithelial  cells. 

The  extent  of  the  mesenchymal  condensation  to  form  the 
muscularis  uteri  also  produces  a  modification  of  the  relations  of  the 
inguinal  ligament  in  the  female.  For  the  ligament  becomes  for  a 
short  portion  of  its  length  included  in  the  condensation  and  thus 
attached  to  the  upper  portion  of  the  uterus.  It  is  consequently 
divided  into  two  portions,  one  extending  from  the  lower  pole  of 


358 


THE    GENITAL   DUCTS 


the  ovary  to  the  uterus  and  forming  the  ligamentum  ovarii  proprium 
and  the  other  extending  from  the  uterus  to  the  anterior  abdominal 
wall  and  forming  what  is  known  in  the  adult  as  the  round  ligament 
of  the  uterus. 

The  diagram,  Fig.  218,  illustrates  the  transformation  from  the 
indifferent  condition  which  occurs  in  the  two  sexes,  and  that  the 


UM 


female  indifferent  male 

Fig.  218. — Diagrams  Illustrating  the  Transformation  of  the  Mullerian  and 

Wolffian  Ducts. 
B,  Bladder;  C,  clitoris;  CG,  canal  of  Gaertner;  CI,  cloaca;  Eo,  epoophoron;  Ep,  epi- 
didymis; F,  Fallopian  tube;  G,  genital  gland;  HE,  hydatid  of  epididymis;  HM,  hydatid 
of  Morgagni;  K,  kidney;  MD,  Mullerian  duct;  O,  ovary;  P,  penis;  Po,  paroophron;  Pr, 
prostate  gland;  R,  rectum;  T,  testis;  U,  urethra;  UM,  uterus  masculinus;  Ur,  ureter; 
US,  urogenital  sinus;  Ut,  uterus;  V,  vagina;  Va,  vas  aberrans;  VD,  vas  deferens;  VS, 
vesicula  seminalis;  WB,  Wolffian  body;  WD,  Wolffian  duct. — (Modified  from  Huxley.) 


homologies  of  the  various  parts  may  be  clearly  understood  they 
may  also  be  stated  in  tabular  form  as  on  the  next  page. 


THE    BLADDER 


359 


Indifferent  Stage. 

Male.                                           Female. 

Genital  ridge   J 

Testis. 

Fimbria  ovarica. 
Ovary. 

Ovarian  ligament. 
Round  ligament. 

Wolffian  body < 

Globus  major  of  epididymis. 

Paradidymis. 

Vasa  aberrantia. 

Epoophoron. 
Paroophoron. 

Wolffian  ducts.  .  .  .   • 

Body    and    globus    minor    of 

epididymis. 
Vasa  deferentia. 
Seminal  vesicles.' 
Ejaculatory  ducts. 

Collecting    tubules    of    epo- 
ophoron. 

Canal  of  Gartner. 

Miillerian  ducts  .  . .  < 

Sessile  hyatid. 
Uterus  masculinus. 

Fallopian  tubes. 

Uterus. 

Vagina. 

In  addition  to  the  sessile  hydatid,  a  stalked  hydatid  also  occurs  in 
connection  with  the  testis,  and  a  similar  structure  is  attached  to  the 
fimbriated  opening  of  each  Fallopian  tube.  The  significance  of  these 
structures  is  uncertain,  though  it  has  been  suggested  that  they  are  per- 
sisting rudiments  of  the  pronephros. 

A  failure  of  the  development  of  the  various  parts  just  described  to  be 
completed  in  the  normal  manner  leads  to  various  abnormalities  in  con- 
nection with  the  reproductive  organs.  Thus  there  may  occur  a  failure 
in  the  fusion  of  the  lower  portions  of  the  Miillerian  ducts,  a  bihorned  or 
bipartite  uterus  resulting,  or  the  two  ducts  may  come  into  contact  and 
their  adjacent  walls  fail  to  disappear,  the  result  being  a  median  partition 
separating  the  vagina  or  both  the  vagina  and  uterus  into  two  compart- 
ments. The  excessive  development  of  the  fold  which  gives  rise  to  the 
hymen  may  lead  to  a  complete  closure  of  the  lower  opening  of  the 
vagina,  while,  on  the  other  hand,  a  failure  of  the  Miillerian  ducts  to 
fuse  may  produce  a  biperforate  hymen. 

The  Development  of  the  Urinary  Bladder  and  the  Uro- 
genital Sinus. — So  far  the  relations  of  the  lower  ends  of  the  urino- 
genital  ducts  have  not  been  considered  in  detail,  although  it  has  been 


s6o 


THE    BLADDER 


seen  that  in  the  early  stages  of  development  the  Wolffian  and 
Miillerian  ducts  open  into  the  sides  of  the  ventral  portion  of  the 
cloaca;  that  the  ureters  communicate  with  the  lower  portions  of  the 
Wolffian  ducts;  that  from  the  ventral  anterior  portion  of  the  cloaca 
the  allantoic  duct  extends  outward  into  the  belly-stalk;  and,  finally 
(p.  281),  that  the  cloaca  becomes  divided  into  a  dorsal  portion,  which 
forms  the  lower  part  of  the  rectum,  and  a  ventral  portion,  which  is 
continuous  with  the  allantois  and  receives  the  urinogenital  ducts 


Fig.  219. — Reconstruction  of  the  Cloacal  Region  op  an  Embryo  of  14  mm. 

al,  Allantois;  b,  bladder;  gt,  genital  tubercle;  i,  intestine;  n,  spinal  cord;  nc,  notochord ; 

r,  rectum;  sg,  urogenital  sinus;  ur,  ureter;  w,  Wolffian  duct.— (Keibel.) 

(Fig.  219).     It  is  the  history  of  this  ventral  portion  of  the  cloaca 
which  is  now  to  be  considered. 

It  may  be  regarded  as  consisting  of  two  portions,  an  anterior  and 
a  posterior,  the  line  of  insertion  of  the  urinogenital  ducts  marking  the 
junction  of  the  two.  The  anterior  or  upper  portion  is  destined  to 
give  rise  to  the  urinary  bladder  (Fig.  219,  b),  while  the  lower  one 
forms  what  is  known  for  a  time  as  the  urogenital ^inus  (sg).  The 
bladder,  when  first  differentiated,  is  a  tubular  structure,  whose 
lumen  is  continuous  with  that  of  the  allantois,  but  after  the  second 


THE    BLADDER 


361 


month  it  enlarges  to  become  more  sac-like,  while  the  intra-embryonic 
portion  of  the  allantois  degenerates  to  a  solid  cord  extending  from 
the  apex  of  the  bladder  to  the  umbilicus  and  is  known  as  the  urachus. 
During  the  enlargement  of  the  bladder  the  terminal  portions  of  the 
urinogenital  ducts  are  taken  up  into  its  walls,  a  process  which 
continues  until  finally  the  ureters  and  Wolffian  ducts  open  into  it 
separately,  the  ureters  opening  to  the  sides  of  and  a  little  anterior 
to  the  ducts.     This  condition  is  reached  in  embryos  of  about  14  mm. 


Fig.  220. — Reconstruction  of  the  Cloacal  Structures  of  an  Embryo  of  25  mm. 

bl,  Bladder;  m,  Mullerian  duct;  r,  rectum;  sg,  urogenital  sinus;  sy,  symphysis  pubis;  u, 

ureter;  ur,  urethra;  w.  Wolffian  duct. — (Adapted  from  Keibel.) 

(Fig.  219),  and  in  later  stages  the  interval  between  the  two  pairs  of 
ducts  is  increased  (Fig.  220),  resulting  in  the  formation  of  a  short 
canal  connecting  the  lower  end  of  the  bladder  which  receives  the 
ureters  with  the  upper  end  of  the  urogenital  sinus,  into  which  the 
Wolffian  and  Mullerian  ducts  open.  This  connecting  canal  repre- 
sents the  urethra  (Fig.  220,  ur),  or  rather  the  entire  urethra  of  the 
female  and  the  proximal  part  of  that  of  the  male,  since  a  considerable 
portion  of  the  latter  canal  is  still  undeveloped  (see  p.  364).     From 


362  THE    UROGENITAL    SINUS 

this  urethra  there  is  developed,  at  about  the  third  month,  a  series  of 
solid  longitudinal  folds  which  project  upon  the  outer  surface  and 
separate  from  the  urethra  from  above  downward.  These  represent 
the  tubules  of  the  prostate  gland  and  are  developed  in  both  sexes, 
although  they  remain  in  a  somewhat  rudimentary  condition  in  the 
female.  The  muscular  tissue,  so  characteristic  of  the  gland  in  the 
adult  male,  is  developed  from  the  surrounding  mesenchyme  at  a 
later  stage. 

The  bladder  is,  accordingly,  essentially  a  derivative  of  the  cloaca 
and  its  mucous  membrane  is  therefore  largely  of  endodermal  origin. 
Portions  of  the  Wolffian  ducts  which  are  of  mesodermal  origin  are, 
however,  taken  up  into  the  wall  of  the  bladder  and  form  a  portion 
of  it.  The  extent  of  the  portion  so  formed  is  indicated  by  the 
position  of  the  orifices  of  the  ureters  above  and  of  the  ejaculatory 
ducts  below,  and  it  corresponds  therefore  with  what  is  termed  the 
trigonum  vesica  together  with  the  floor  of  the  urethra  as  far  as  the 
openings  of  the  ejaculatory  ducts.  Throughout  this  region  the 
mucous  membrane  is  of  mesodermal  origin. 

The  urogenital  sinus  is  in  the  early  stages  also  tubular  in  its 
upper  part,  though  it  expands  considerably  below,  where  it  is 
closed  by  the  cloacal  membrane.  This,  by  the  separation  of  the 
cloaca  into  rectum  and  sinus,  has  become  divided  into  two  portions, 
the  more  ventral  of  which  closes  the  sinus  and  the  dorsal  the  rectum, 
the  interval  between  them  having  become  considerably  thickened 
to  form  the  perineal  body.  In  embryos  of  about  17  mm.  the  uro- 
genital portion  of  the  membrane  has  broken  through,  and  in  later 
stages  the  tubular  portion  of  the  sinus  is  gradually  taken  up  into 
the  more  expanded  lower  portion,  until  finally  the  entire  sinus  forms 
a  shallow  depression,  termed  the  vestibule,  into  the  upper  part  of 
which  the  urethra  opens,  while  below  are  the  openings  of  the 
Wolffian  (ejaculatory)  ducts  in  the  male  or  the  orifice  of  the  vagina 
in  the  female.  From  the  sides  of  the  lower  part  of  the  sinus  a  pair 
of  evaginations  arise  toward  the  end  of  the  fourth  month  and  give 
rise  to  the  bulbo-vestibular  glands  (Bartholin's)  of  the  female  or  the 
corresponding  bulbo-urethral  glands  (Cowper's)  in  the  male. 


THE    EXTERNAL    GENITALIA  363 

The  Development  of  the  External  Genitalia. — At  about  the 
fifth  week,  before  the  urogenital  sinus  has  opened  to  the  exterior, 
the  mesenchyme  on  its  ventral  wall  begins  to  thicken,  producing  a 
slight  projection  to  the  exterior.  This  eminence,  which  is  known 
as  the  genital  tubercle  (Fig.  219,  gt),  rapidly  increases  in  size,  its 
extremity  becomes  somewhat  bulbously  enlarged  (Fig.  221,  gl)  and 
a  groove,  extending  to  the  base  of  the  terminal  enlargement,  appears 
upon  its  vestibular  surface,  the  lips  of  the  groove  forming  two  well- 
marked  genital  folds  (Fig.  221,  gf).  At  about  the  tenth  week  there 
appears  on  either  side  of  the  tubercle  an  enlargement  termed  the 
genital  swelling  (Fig.  221,  gs),  which  is  due  to  a  thickening  of  the 
mesenchyme  of  the  lower  part  of  the  ventral^abdominal  wall  in  the 


V^ 


Fig.  221. — The  External  Genitalia  of  an  Embryo  of  25  mm. 
a,  Anus;  gf,  genital  fold;  gl,  glans;  gs,  genital  swelling;  p,  perineal  body. — (Keibel.) 

region  where  the  inguinal  ligament  is  attached,  and  with  the  appear- 
ance of  these  structures  the  indifferent  stage  of  the  external  genitals 
is  completed. 

In  the  female  the  growth  of  the  genital  tubercle  proceeds  rather 
slowly  and  it  becomes  transformed  into  the  clitoris,  the  genital  folds 
becoming  prolonged  to  form  the  labia  minora.  The  genital  swellings 
increase  in  size,  their  mesenchyme  becomes  transformed  into  a  mass 
of  adipose  and  fibrous  tissue  and  they  become  converted  into  the 
labia  majora,  the  interval  between  them  constituting  the  vulva. 

In  the  male  the  early  stages  of  development  are  closely  similar  to 


364 


THE    EXTERNAL    GENITALIA 


those  of  the  female;  indeed,  it  has  been  well  said  that  the  external 
genitals  of  the  adult  female  resemble  those  of  the  fetal  male.  In 
early  stages  the  genital  tubercle  elongates  to  form  the  penis  and  the 
integument  which  covers  the  proximal  part  of  it  grows  forward  as  a 
fold  which  encloses  the  bulbous  enlargement  or  glans  and  forms  the 
prepuce,  whose  epithelium  fuses  with  that  covering  the  glans  and 
only  separates  from  it  later  by  a  cornification  of  the  cells  along  the 
plane  of  fusion.  The  genital  folds  meet  together  and  fuse,  converting 
the  vestibule  and  the  groove  upon  the  vestibular  surface  of  the  penis 
into  the  terminal  portion  of  the  male  urethra  and  bringing  it  about 
that  the  vasa  deferentia  and  the  uterus  masculinus  open  upon  the 
floor  of  that  passage.  The  two  genital  swellings  are  at  the  same 
time  brought  closer  together,  so  as  to  lie  between  the  base  of  the 
penis  and  the  perineal  body  and,  eventually,  they  form  the  scrotum. 
The  mesenchyme  of  which  they  were  primarily  composed  differenti- 
ates into  the  same  layers  as  are  found  in  the  wall  of  the  abdomen  and 
a  peritoneal  pouch  is  prolonged  into  them  from  the  abdomen,  so  that 
they  form  sacs  into  which  the  testes  descend  toward  the  close  of  fetal 
life  (p.  366). 

The  homologies  of  the  portions  of  the  reproductive  apparatus 
derived  from  the  cloaca  and  of  the  external  genitalia  in  the  two  sexes 
may  be  perceived  from  the  following  table. 


Male 

Female 

Urinary  bladder. 

Urinary  bladder. 

Proximal  portion  of  urethra. 

Urethra. 

Bulbo-urethral  glands. 

Bulbo-vestibular  glands. 

Urogenital  sinus .... 

The  rest  of  the  urethra. 

Vestibule. 

Genital  tubercle.  .  .  . 

Penis. 

Clitoris. 

Genital  folds 

Prepuce  and  integument  of 

penis. 

Labia  minora 

Genital  swellings...  . 

Scrotum. 

Labia  majora. 

It  is  stated  above  that  the  layers  which  compose  the  walls  of  the  scro- 
tum are  identical  with  those  of  the  abdominal  wall.  This  may  be  seen  in 
detail  from  the  following  scheme: 


THE    DESCENT    OF   THE    OVARIES  365 

Abdominal  Walls.  Scrotum. 

Integument.  Integument. 

Superficial  fascia.  Dartos. 

External  oblique  muscle.  Intercolumnar  fascia. 

Internal  oblique  muscle.  Cremasteric  fascia. 

Transverse  muscle.  Infundibuliform  fascia. 

Peritoneum.  Tunica  vaginalis. 

Numerous  anomalies,  depending  upon  an  inhibition  or  excess  of  the 
development  of  the  parts,  may  occur  in  connection  with  the  external 
genitalia.  Should,  for  instance,  the  lips  of  the  groove  on  the  vestibular 
surface  of  the  penis  fail  to  fuse,  the  penial  portion  of  the  urethra  remains 
incomplete,  constituting  a  condition  known  as  hypospadias,  a  condition 
whic,h  offers  a  serious  bar  to  the  fulfilment  of  the  sexual  act.  If  the 
hypospadias  is  complete  and  there  be  at  the  same  time  an  imperfect 
development  of  the  penis,  as  frequently  occurs  in  such  cases,  the  male 
genitalia  closely  resemble  those  of  the  female  and  a  condition  is  produced 
which  is  usually  known  as  hermaphroditism.  It  is  noteworthy  that  in 
such  cases  there  is  frequently  a  somewhat  excessive  development  of  the 
uterus  masculinus,  and  a  similar  condition  may  be  produced  in  the 
female  by  an  excessive  development  of  the  clitoris.  Such  cases,  however, 
which  concern  only  the  accessory  organs  of  reproduction,  are  instances  of 
what  is  more  properly  termed  spurious  hermaphroditism,  true  hermaph- 
roditism being  a  term  which  should  be  reserved  for  possible  cases  in 
which  the  genital  ridges  give  rise  in  the  same  individual  to  both  ova  and 
spermatozoa.  Such  cases  are  of  exceeding  rarity  in  the  human  species, 
although  occasionally  observed  in  the  lower  vertebrates,  and  the  great 
majority  of  the  examples  of  hermaphroditism  hitherto  observed  are  cases 
of  the  spurious  variety. 

The  Descent  of  the  Ovaries  and  Testes. — The  positions 
finally  occupied  by  the  ovaries  and  testes  are  very  different  from 
those  which  they  possess  in  the  earlier  stages  of  development,  and 
this  is  especially  true  in  the  case  of  the  testes.  The  change  of  position 
is  partly  due  to  the  rate  of  growth  of  the  inguinal  ligaments  being 
less  than  that  of  the  abdominal  walls,  the  reproductive  organs  being 
thereby  drawn  downward  toward  the  inguinal  regions  where  the 
ligaments  are  attached.  The  point  of  attachment  is  beneath  the 
bottom  of  a  slight  pouch  of  peritoneum  which  projects  a  short  dis- 
tance into  the  substance  of  the  genital  swellings  and  is  known  as  the 
canal  of  Nuck  in  the  female,  and  in  the  male  as  the  vaginal  process. 

In  the  female  a  second  factor  combines  with  that  just  mentioned. 


366 


THE    DESCENT    OF   THE    TESTES 


The  relative  shortening  of  the  inguinal  ligaments  acting  alone 
would  draw  the  ovaries  toward  the  inguinal  regions,  but  since  they 
are  united  to  the  uterus  by  the  ovarian  ligaments  movement  in  that 
direction  is  prevented  and  the  ovaries  come  to  lie  in  the  recto-uterine 
compartment  of  the  pelvic  cavity. 

With  the  testes  the  case  is  more  complicated,  since  in  addition  to 
the  relative  shortening  of  the  inguinal  ligaments  there  is  an  elonga- 
tion of  the  vaginal  processes  into  the  substance  of  the  genital  swell- 
ings, and  it  must  be  remembered  that  the  testes,  like  the  ovaries,  are 
primarily  connected  with  the  peritoneum.  Three  stages  may  be 
recognized  in  the  descent  of  the  testes.     The  first  of  these  depends 


Fig.  222. — Diagrams  Illustrating  the  Descent  of  the  Testis. 
il,  Inguinal  ligament;  m,  muscular  layer;  s,  skin  and  dartos  of  the  scrotum;  t,  testis; 
tv,  tunica  vaginalis ;  vd,  vas  deferens ;  vp,  vaginal  process  of  peritoneum. — (After  Hertwig.) 

on  the  slow  rate  of  elongation  of  the  inguinal  ligaments  or  guber- 
nacula.  It  lasts  until  about  the  fifth  month  of  development,  when 
the  testes  lie  in  the  inguinal  region  of  the  abdomen,  but  during  this 
month  the  elongation  of  the  gubernaculum  becomes  more  rapid  and 
brings  about  the  second  stage,  during  which  there  is  a  slight  ascent 
of  the  testes,  so  that  they  come  to  lie  a  little  higher  in  the  abdomen. 
This  stage  is,  however,  of  short  duration,  and  is  succeeded  by  the 
stage  of  the  final  descent,  which  is  characterized  by  the  elongation 
of  the  vaginal  processes  of  the  peritoneum  into  the  substance  of  the 
scrotum  (Fig.  222,  A).     Since  the  gubernaculum  is  attached  to  the 


THE    DESCENT    OF   TBE    TESTES  367 

abdominal  wall  beneath  this  process,  and  since  its  growth  has  again 
diminished,  the  testes  gradually  assume  again  their  inguinal  position, 
and  are  finally  drawn  down  into  the  scrotum  with  the  vaginal 
processes. 

The  condition  which  is  thus  acquired  persists  for  some  time  after 
birth,  the  testicles  being  readily  pushed  upward  into  the  abdominal 
cavity  along  the  cavity  by  which  they  descended.  Later,  however, 
the  size  of  the  openings  of  the  vaginal  processes  into  the  general 
peritoneal  cavity  becomes  greatly  reduced,  so  that  each  process 
becomes  converted  into  an  upper  narrow  neck  and  a  lower  sac-like 
cavity  (Fig.  222,  B),  and,  still  later,  the  walls  of  the  neck  portion  fuse 
and  become  converted  into  a  solid  cord,  while  the  lower  portion, 
wrapping  itself  around  the  testis,  becomes  the  tunica  vaginalis  (tv). 
By  these  changes  the  testes  become  permanently  located  in  the  scro- 
tum. During  the  descent  of  the  testes  the  remains  of  each  Wolffian 
body,  the  epididymis,  and  the  upper  part  of  each  vas  deferens 
together  with  the  spermatic  vessels  and  nerves,  are  drawn  down  into 
the  scrotum,  and  the  mesenterial  fold  in  which  they  were  originally 
contained  also  practically  disappears,  becoming  converted  into  a 
sheath  of  connective  tissue  which  encloses  the  vas  deferens  and  the 
vessels  and  nerves,  binding  them  together  into  what  is  termed  the 
spermatic  cord.  The  mesorchium,  which  united  the  testis  to  the 
peritoneum  enclosing  the  Wolffian  body,  does  not  share  in  the  degen- 
eration of  the  latter,  but  persists  as  a  fold  extending  between  the 
epididymis  and  the  testis  and  forming  the  sinus  epididymis. 

In  the  text-books  of  anatomy  the  spermatic  cord  is  usually  described 
as  lying  in  an  inguinal  canal  which  traverses  the  abdominal  walls  obliquely 
immediately  above  Poupart's  ligament.  So  long  as  the  lumen  of  the  neck 
portion  of  the  vaginal  process  of  peritoneum  remains  patent  there  is  such 
a  canal,  placing  the  cavity  of  the  tunica  vaginalis  in  communication  with 
the  general  peritoneal  cavity,  but  the  cord  does  not  traverse  this  canal, 
but  lies  outside  it  in  the  retroperitoneal  connective  tissue.  When, 
however,  the  neck  of  the  vaginal  process  disappears,  a  canal  no  longer 
exists,  although  the  connective  tissue  which  surrounds  the  spermatic 
cord  and  unites  it  with  the  tissues  of  the  abdominal  walls  is  less  dense  than 
the  neighboring  tissues,  so  that  the  cord  may  readily  be  separated  from 
these  and  thus  appear  to  He  in  a  canal. 


368  LITERATURE 

LITERATURE. 

B.  M.  Allen:  "The  Embryonic  Development  of  the  Ovary  and  Testes  in  Mammals," 

Amer.  Journ.  of  AnaL,  in,  1904. 
J.  L.  Bremer:  "Morphology  of  the  Tubules  of  the  Human  Testis  and  Epididymis," 

Amer.  Journ.  Anat.,  xi,  1911. 
E.  J.  Evatt:  "A  Contribution  to  the  Development  of  the  Prostate  in  Man,"  Journ. 

Anat.  and  Phys.,  xliii,  1909. 
E.  J.  Evatt:  "  A  Contribution  to  the  Development  of  the  Prostate  Gland  in  the  Human 

Female,"  Journ.  Anat.  and  Phys.,  xlv,  1911. 
W.  Felix:  " Entwickelungsgeschichte  des  Exkretions-sy stems,"  Ergebn.  der  Anat.  und 

Entwicklungsgesch.,  xni,  1903. 
W.  Felix:  "Die  Entwicklung  der  Ham-  und  Geschlechtsorgane,"  in  Keibel-Mall 

Human  Embryology,  II,  1912. 
A.  Fleischmann:  "  Morphologische  Studien  liber  Kloake  und  Phallus  der  Amnioten, 

Morphol.  Jarhbuch,  xxx,  xxxii  und  xxxvi,  1902,  1904,  1907. 
O.  Frankl:  "Beitrage  zur  Lehre  vom  Descensus  testiculorum,"  Sitzungsber.  der  kais. 

Akad.  Wissensch.  Wien,  Math.-Naturwiss.  Classe,  cix,  1900. 
S.  P.  Gage:  "A  Three  Weeks  Human  Embryo,  with  especial  reference  to  the  Brain 

and  the  Nephric  System,"  Amer.  Journ.  of  Anat.,  rv,  1905. 
D.  B.  Hart:  "  The  Nature  and  Cause  of  the  Physiological  Descent  of  the  Testes," 

Journ.  Anat.  and  Phys.,  xliv,  1909. 

D.  B.  Hart:  "  The  Physiological  Descent  of  the  Ovaries  in  the  HumanFoetus,"     Journ. 

Anat.  and  Phys.,  xliv,  1909. 

E.  Hauch:  "Ueber  die  Anatomie  und  Entwicklung  der  Nieren,"  Anat.  Hefte,  xxii, 

1903. 
G.  C.  Huber:  "On  the  Development  and  Shape  of  the  Uriniferous  Tubules  of  Certain 

of  the  Higher  Mammals,"  Amer.  Journ.  of  Anat.,  rv,  Suppl.  1905. 
J.  Janosik:  "Histologisch-embryologische  Untersuchungen  uber  das  Urogenitalsystem," 

Sitzungsber.  der  kais.  Akad.  Wissensch.  Wien,  Math.-Naturwiss.  Classe,  xci,  1887 
J.  Janosik:  "Ueber  die  Entwicklung  der  Nachniere  bei  den  Amnioten,"  Arch,  fur 

Anat.  u.  Phys.,  Anat.  Abth.,  1907. 
J.  Janosik:  "Entwicklung  des  Nierenbeckens  beim  Menschen,"  Arch,  fitr  mikrosk. 

Anat.,  lxxviii,  191 1. 

F.  Keibel:  "Zur  Entwickelungsgeschichte  des  menschlichen  Urogenital-apparatus," 

Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1896. 
J.  B.  Macallum:  "Notes  on  the  Wolffian  Body  of  Higher  Mammals,"  Amer.  Journ.  0 

Anat.,  1,  1902. 
E.  Martin:  "Ueber  die  Anlage  der  Urniere  beim  Kaninchen,"  Archiv  fiir  Anat.  und 

Physiol.,  Anat.  Abth.,  1888. 
H.  Meyer:  "Die  Entwickelung  der  Urnieren  beim  Menschen,"  Archiv  fiir  mikrosk. 

Anat.,  xxxvi,  1890. 
R.  Meyer:  "Zur  Kenntnis  des  Gartner'schen  Ganges  besonders  in  der  Vagina  und 

dem  Hymen  des  Menschen,"  Arch,  fur  mikrosk.  Anat.,  lxxiii,  1909. 
R.  Meyer:  "Zur  Entwicklungsgeschichte  und  Anatomie  des  utriculus  prostaticus  beim 

Menschen,"  Arch,  fiir  mikrosk.  Anat.,  lxxtv,  1909 


LITERATURE  369 

G.  VON   Mihalkovicz  :    "  Untersuchungen  iiber   die   Entwickelung   des   Ham-   und 

Geschlechtsapparates    der    Amnioten,"    Internat.    Monatsschrift  fiir    Anat.   und 

Physiol.,  11,  1885. 
W.  Nagel:  "Ueber  die  Entwickelung  des  Urogenitalsystems  des  Menschen,"  Archiv 

fiir  mikros.  Anat.,  xxxiv,  1889. 
W.  Nagel:  "Ueber  die  Entwickelung  des  Uterus  und  der  Vagina  beim  Menschen," 

Archiv  fiir  mikros k.  Anat.,  xxxvn,  1891. 
W.  Nagel:  "Ueber  die  Entwickelung  der  innere  und  aussere  Genitalien  biem  mensch- 

lichen  Weibes,"  Archiv  fiir  Gynakol.,  xlv,  1894. 
K.  Peter:  "Untersuchungen  iiber  Bau  und  Entwicklung  der  Niere.  I.  Die  Nieren- 

kanalchen  des  Menschen  und  einiger  Saugetiere,  Jena,  1909. 
A.  G.  Pohlman:  "The  Development  of  the  Cloaca  in  Human  Embryos."  Amer.  Journ. 

of  Anat.,  xii,  191 1. 
W.  Rubaschkin:  "  Ueber  die  Urgeschlechtszellen  bei  Saugetiere,'Mwa<.  Hefte,  xxxix, 

1909. 
K.  E.  Schrelner:  "Ueber  die  Entwicklung  der  Amniotenniere,"  Zeit.  fiir  wissensch. 

Zool.,  lxxi,  1902. 
O.  Stoerk:  "Beitrag  zur  Kenntnis  des  Aufbaues  der  menschlichen  Niere,"  Anat. 

Hefte,  xxill,  1904. 
J.  Tandler:  "Ueber  Vornieren-Rudimente  beim  menschliche  Embryo,"  Anat.  Hefte, 

xxvni,  1905. 
F.  J.  Taussig:  "The  Development  of  the  Hymen,"  Amer.  Journ.  Anat.,  viii,  1908. 
F.  Tourneux:  "  Sur  le  developpement  et  revolution  du  tubercule  genital  chez  le  foetus 

humain  dans  les  deux  sexes,"  Journ.  de  I' Anat.  et  de  la  Physiol.,  xxv,  1889. 
S.  Weber:  "  Zur  Entwickelungsgeschichte  des  uropoetischen  Apparates  bei  Saugern, 

mit  besonderer  Beriicksichtigung  der  Urniere  zur  Zeit  des  Auftretens  der  blei- 

benden  Niere,"  Morphol.  Arbeiten,  vil,  1897. 


24 


CHAPTER  XIV. 
THE  SUPRARENAL  SYSTEM  OF  ORGANS. 

To  the  suprarenal  system  a  number  of  bodies  of  peculiar  struc- 
ture, probably  concerned  with  internal  secretion,  may  be  assigned. 
In  the  fishes  they  fall  into  two  distinct  groups,  the  one  containing 
organs  derived  from  the  ccelomic  epithelium  and  known  as  intervened 
organs,  and  the  other  consisting  of  organs  derived  from  the  sym- 
pathetic nervous  system  and  which,  on  account  of  the  characteristic 
affinity  they  possess  for  chromium  salts,  have  been  termed  the 
chroma ffine  organs.  But  in  the  amphibia  and  amniote  vertebrates, 
while  both  the  groups  are  represented  by  independent  organs,  yet 
they  also  become  intimately  associated  to  form  the  suprarenal  bodies, 
so  that,  notwithstanding  their  distinctly  different  origins,  it  is 
convenient  to  consider  them  together. 

The  Development  of  the  Suprarenal  Bodies. — The  supra- 
renal bodies  make  their  appearance  at  an  early  stage,  while  the 
Wolffian  bodies  are  still  in  a  well-developed  condition,  and  they  are 
situated  at  first  to  the  medial  side  of  the  upper  ends  of  these  struc- 
tures (Fig.  211,  sr).  Their  final  relation  to  the  metanephros  is  a 
secondary  event,  and  is  merely  a  topographic  relation,  there  being 
no  developmental  relation  between  the  two  structures. 

In  the  human  embryo  they  make  their  appearance  at  about  the 
beginning  of  the  fourth  week  of  development  as  a  number  of  pro- 
liferations of  the  ccelomic  epithelium,  which  project  into  the  sub- 
jacent mesenchyme,  and  are  situated  on  either  side  of  the  median 
line  between  the  root  of  the  mesentery  and  the  upper  portion  of  the 
Wolffian  body.  The  various  proliferations  soon  separate  from  the 
epithelium  and  unite  to  form  two  masses  situated  in  the  mesenchyme, 
one  on  either  side  of  the  upper  portion  of  the  abdominal  aorta.  In 
certain  forms,  such  as  the  rabbit,  the  primary  proliferations  arise 

37° 


DEVELOPMENT    OF    THE    SUPRARENAL    BODIES 


371 


from  the  bottom  of  depressions  of  the  ccelomic  epithelium  (Fig.  223), 
but  in  the  human  embryo  these  depressions  do  not  form. 

Up  to  this  stage  the  structure  is  a  pure  interrenal  organ,  but 
during  the  fifth  week  of  development  masses  of  cells,  derived  from 
the  abdominal  portion  of  the  sympathetic  nervous  system,  begin  to 
penetrate  into  each  of  the  interrenal  masses  (Fig.  224),  and  form 
strands  traversing  them.  At  about  the  ninth  or  tenth  week  fatty 
granules  begin  to  appear  in  the  interrenal  cells  and  somewhat  later, 
about  the  fourth  month,  the  sympathetic  constituents  begin  to  show 
their  chromaffine  characteristics.  The  two  tissues,  however,  remain 
intermingled  for  a  considerable  time,  and  it  is  not  until  a  much  later 


Ao 


&  Sr  ns 

tern         J  „-.-— 


WC 


-fvd 


Fig.  223. — Section  through  a  Portion  of  the  Wolffian  Ridge  of  a  Rabbit 

Embryo  of  6.5  mm. 

Ao,  Aorta;  ns,  nephrostome;  Sr,  suprarenal  body;  vc,  cardinal  vein;  wc,  tubule  of 

Wolffian  body;  wd,  Wolffian  duct. — (Aichel.) 

period  that  they  become  definitely  separated,  the  sympathetic 
elements  gradually  concentrating  in  the  center  of  the  compound 
organ  to  become  its  medullary  substance,  while  the  interrenal  tissue 
forms  the  cortical  substance.  Indeed,  it  is  not  until  after  birth  that 
the  separation  of  the  two  tissues  and  their  histological  differentiation 
is  complete,  occasional  masses  of  interrenal  tissue  remaining 
imbedded  in  the  medullary  substance  and  an  immigration  of 
sympathetic  cells  continuing  until  at  least  the  tenth  year  (Wiesel). 

A  great  deal  of  difference  of  opinion  has  existed  in  the  past  concerning 
the  origin  of  the  suprarenal  glands.  By  several  authors  they  have  been 
regarded  as  derivatives  in  whole  or  in  part  of  the  excretory  apparatus, 
some  tracing  their  origin  to  the  mesonephros  and  others  even  to  the  pro- 
nephros.    The  fact  that  in  some  mammals  the  cortical  (interrenal)  cells  are 


372       DEVELOPMENT  OF  THE  SUPRARENAL  BODIES 

formed  from  the  bottom  of  depressions  of  the  coelomic  epithelium  seemed 
to  lend  support  to  this  view,  but  it  is  now  pretty  firmly  established  that 
the  appearances  thus  presented  do  not  warrant  the  interpretation  placed 
upon  them  and  that  the  interrenal  tissue  is  derived  from  the  ccelomic 
epithelium  quite  independently  of  the  nephric  tubules.  That  the  chrom- 
affine  tissue  is  a  derivative  of  the  sympathetic  nervous  system  has  long 
been  recognized. 

During  the  development  of  the  suprarenal  glands  portions  of 
their  tissue  may  be  separated  as  the  result  of  unequal  growth  and 
form  what  are  commonly  spoken  of  as  accessory  suprarenal  glands, 
although,  since  they  are  usually  composed  solely  of  cortical  sub- 


...  ■  / 

'•'§'''•.    ' .  .     .■/. 


$M 


S.B. 

Fig.  224. — Section  through  the  Suprarenal  Body  of  an  Embryo  of  17  mm. 

A,  Aorta;  R,  interrenal  portion;  S,  sympathetic  nervous  system;  SB,  sympathetic  cells 

penetrating  the  interrenal  portion. — (Wiesel.) 

stance,  the  term  accessory  interrenal  bodies  would  be  more  appropriate. 
They  may  be  formed  at  different  periods  of  development  and  occur 
in  various  situations,  as  for  instance,  in  the  vicinity  of  the  kidneys 
or  even  actually  imbedded  in  their  substance,  on  the  walls  of  neigh- 
boring blood-vessels,  in  the  retroperitoneal  tissue  below  the  level  of 
the  kidneys,  and  in  connection  with  the  organs  of  reproduction,  in 
the  spermatic  cord,  epididymis  or  rete  testis  of  the  male  and  in  the 
broad  ligament  of  the  female. 

It  seems  probable  that  the  bodies  associated  with  the  reproductive 


DEVELOPMENT   OF   THE    SUPRARENAL   BODIES 


373 


apparatus  are  separated  from  the  main  mass  of  interrenal  tissue 
before  the  immigration  of  the  sympathetic  tissue  and  before  the 
descent  of  the  ovaries  or  testes,  while  those  which  occur  at  higher 
levels  are  of  later  origin,  and  in  some  cases  may  contain  some  med- 
ullary substance,  being  then  true  accessory  suprarenals.  Such 
bodies  are,  however,  comparatively  rare,  the  great  majority  of  the 
accessory  bodies  being  composed  of  interrenal  tissue  alone. 

Independent  chromamne  organs  also  occur,  among  them  the 


Fig.  225. — Section  of  a  Cell  Ball  from  the  Intercarotid  Ganglion  of  Man 

be,  Blood  capillaries;  ev,  efferent  vein;  S,  connective-tissue  septum;  I,  trabecular — 

(From  Bohm  and  Davidoff,  after  Schaper.) 


intercarotid  ganglia  and  the  organs  of  Zuckerkandl  being  especially 
deserving  of  note.  It  may  also  be  pointed  out,  however,  that  the 
chromamne  cells  have  the  same  origin  as  the  cells  of  the  sympathetic 
ganglia  and  may  sometimes  fail  to  separate  from  the  latter,  so  that 
the  sympathetic  ganglia  and  plexuses  frequently  contain  chromamne 
cells. 

The   Intercarotid  Ganglia. — These   structures,    which    are   fre- 


374  TTTF.    INTERCAROTID    GANGLIA 

quently  though  incorrectly  termed  carotid  glands,  are  small  bodies 
about  5  mm.  in  length,  which  lie  usually  to  the  mesial  side  of  the 
upper  ends  of  the  common  carotid  arteries.  They  possess  a  very 
rich  arterial  supply  and  stand  in  intimate  relation  with  the  branches 
of  an  intercarotid  sympathetic  plexus,  and,  furthermore,  they  are 
characterized  by  possessing  as  their  specific  constituents  markedly 
chromamne  cells,  among  which  are  scattered  stellate  cells  resembling 
the  cells  of  the  sympathetic  ganglia. 

They  have  been  found  to  arise  in  pig  embryos  of  44  mm.  by  the 
separation  of  cells  from  the  ganglionic  masses  scattered  throughout 
the  carotid  sympathetic  plexuses.  These  cells,  which  become  the 
chromamne  cells,  arrange  themselves  in  round  masses  termed  cell 
balls,  many  of  which  unite  to  form  each  ganglion,  and  in  man  each 
cell  ball  becomes  broken  up  into  trabecule  by  the  blood-vessels 
(Fig.  225)  which  penetrate  its  substance,  and  the  individual  balls  are 
separated  from  one  another  by  considerable  quantities  of  connective 
tissue. 

Some  confusion  has  existed  in  the  past  as  to  the  origin  of  this  structure. 
The  mesial  wall  of  the  proximal  part  of  the  internal  carotid  artery  becomes 
considerably  thickened  during  the  early  stages  of  development  and  the 
thickening  is  traversed  by  numerous  blood  lacunae  which  communicate 
with  the  lumen  of  the  vessel.  This  condition  is  perhaps  a  relic  of  the 
branchial  capillaries  which  in  the  lower  gill-breathing  vertebrates  repre- 
sent the  proximal  portion  of  the  internal  carotid,  and  has  nothing  to  do 
with  the  formation  of  the  intercarotid  ganglion,  although  it  has  been 
believed  by  some  authors  (Schaper)  that  the  ganglion  was  derived  from 
the  thickening  of  the  wall  of  the  vessel.  The  fact  that  in  some  animals, 
such  as  the  rat  and  the  dog,  the  ganglion  stands  in  relation  with  the 
external  carotid  and  receives  its  blood- supply  from  that  vessel  is  of  im- 
portance in  this  connection. 

The  thickening  of  the  internal  carotid  disappears  in  the  higher 
vertebrates  almost  entirely,  but  in  the  Amphibia  it  persists  throughout 
life,  the  lumen  of  the  proximal  part  of  the  vessel  being  converted  into  a 
fine  meshwork  by  the  numerous  trabecular  which  traverse  it.  This 
carotid  labyrinth  has  been  termed  the  carotid  gland,  a  circumstance 
which  has  probably  assisted  in  producing  confusion  as  to  the  real  signifi- 
cance of  the  intercarotid  ganglion. 

The   Organs  of  Zuckerkandl. — In  embryos   of   14.5   mm.   there 
have  been  found,  in  front  of  the  abdominal  aorta,  closely  packed 


THE    ORGANS    OF    ZUCKERKANDL 


375 


groups  of  cells  which  resemble  in  appearance  the  cells  composing 
the  ganglionated  cord,  two  of  these  groups,  which  extend  downward 
along  the  side  of  the  aorta  to  below  the  point  of  origin  of  the  inferior 
mesenteric  artery,  being  especially  distinct.  These  cell  groups  give 
rise  to  the  ganglia  of  the  prevertebral  sympathetic  plexuses  and  also 


Fig.  226. — Organs  of  Zuckerkandl  from  a  New-born  Child. 
a,  Aorta;  ci,  inferior  vena  cava;  i.c,  common  iliac  artery;  mi,  inferior  mesenteric 
artery;  n.l  and  n.r,  left  and  right  accessory  organs;  pl.a,  aortic  plexus;  u,  ureter;  v.r.s, 
left  renal  vein. — (Zuckerkandl.) 

to  peculiar  bodies  which,  from  their  discoverer,  may  be  termed  the 
organs  of  Zuckerkandl.  Each  body  stands  in  intimate  relation  with 
the  fibers  of  the  sympathetic  plexuses  and  has  a  rich  blood-supply, 
resembling  in  these  respects  the  intercarotid  ganglia,  and  the  resem- 


376  LITERATURE 

blance  is  further  increased  by  the  fact  that  the  specific  cells  of  the 
organ  are  markedly  chromamne. 

i    At  birth  the  bodies  situated  in  the  upper  portion  of  the  abdominal 
cavity  have  broken  up  into  small  masses,  but  the  two  lower  ones, 
mentioned  above,  are  still  well  defined  (Fig.  226).     Even  these,  how-# 
ever,  seem  to  disappear  later  on  and  no  traces  of  them  have  as  yet 
been  found  in  the  adult. 

LITERATURE. 

A.  Kohn:  "Ueber  den  Bau  und  die  Entwickelung  der  sog.  Carotisdruse,"  Archiv. 

fur  mikrosk.  Anat.,  lvi,  1900. 
A.  Kohn:  "Das  chromaffine  Gewebe,"  Ergebn.  der  Anat.  und  Entwickelungsgesch., 

xii,  1902. 
H.  Poll:  "Die  vergleichende  Entwicklungsgeschichte  der  Nebennierensysteme  der 

Wirbeltiere,"  Hertwig's  Handb.  der  vergl.  und  exper.  Entwicklungslehre  der  Wirbel- 

tiere,  in,  1906. 
A.   Sotjlie:    "Recherches   sur   le   developpement  des   capsules   surrenales   chez   les 

Vertebres,"  Journ.  de  V Anat.  et  de  la  Physiol.,  xxxix,  1903. 
J.  Wiesel:  "Beitrage  zur  Anatomie  und  Entwickelung  der  menschlichen  Nebenniere," 

Anat.  Heft.,  xix,  1902. 
E.  Zuckerkandl:    "Ueber  Nebenorgane  des  Sympathicus  im  Retroperitonealraum 

des  Menschen,"  Verhandl.  Anat.  Gesellsch.,  xv,  1901. 


CHAPTER  XV. 

THE  DEVELOPMENT  OF  THE  NERVOUS 
SYSTEM. 

The  Histogenesis  of  the  Nervous  System. — The  entire  central 
nervous  system  is  derived  from  the  cells  lining  the  medullary  groove, 
whose  formation  and  conversion  into  the  medullary  canal  has  already 
been  described  (p.  72).  When  the  groove  is  first  formed,  the  cells 
lining  it  are  somewhat  more  columnar  in  shape  than  those  on  either 
side  of  it,  though  like  them  they  are  arranged  in  a  single  layer; 
later  they  increase  by  mitotic  division  and  arrange  themselves  in 
several  layers,  so  that  the  ectoderm  of  the  groove  becomes  very  much 
thicker  than  that  of  the  general  surface  of  the  body.  At  the  same 
time  the  cell  boundaries,  which  were  originally  quite  distinct, 
gradually  disappear,  the  tissue  becoming  a  syncytium.  While  its 
tissue  is  in  this  condition  the  lips  of  the  medullary  groove  unite, 
and  the  subsequent  differentiation  of  the  canal  so  formed  differs 
somewhat  in  different  regions,  although  a  fundamental  plan  may  be 
recognized.  This  plan  is  most  readily  perceived  in  the  region  which 
becomes  the  spinal  cord,  and  may  be  described  as  seen  in  that  region. 

Throughout  the  earlier  stages,  the  cells  lining  the  inner  wall  of 
the  medullary  tube  are  found  in  active  proliferation,  some  of  the 
cells  so  produced  arranging  themselves  with  their  long  axes  at  right 
angles  to  the  central  canal  (Fig.  227),  while  others,  whose  destiny 
is  for  the  most  part  not  yet  determinable,  and  which  therefore  may 
be  termed  indifferent  cells  are  scattered  throughout  the  syncytium. 
At  this  stage  a  transverse  section  of  the  medullary  tube  shows  it  to 
be  composed  of  two  well-defined  zones,  an  inner  one  immediately 
surrounding  the  central  canal  and  composed  of  the  indifferent  cells 
and  the  bodies  of  the  inner  or  ependymal  cells,  and  an  outer  one  con- 
sisting of  branched  prolongations  of  the  syncytial  cytoplasm.     This 

377 


378  THE    HISTOGENESIS    OF   THE    NERVOUS    SYSTEM 

outer  layer  is  termed  the  marginal  velum  (Randschleier)  (Fig.  227, 
m).  The  indifferent  cells  now  begin  to  wander  outward  to  form 
a  definite  layer,  termed  the  mantle  layer,  lying  between  the  marginal 
velum  and  the  bodies  of  the  ependymal  cells  (Fig.  228),  and  when 
this  layer  has  become  well  established  the  cells  composing  it  begin 
to  divide  and  to  differentiate  into  (1)  cells  termed  neuroblasts, 
destined  to  become  nerve-cells,  and  (2)  others  which  appear  to  be 
supportive  in  character  and  are  termed  neuroglia  cells  (Fig.  228,  B). 


6r     °    % 


,'.    I"'       «9 


W: 


cs 


Fig.  227. — Transverse  Section  through  the  Spinal  Cord  of  a  Pig  Embryo 
of  30  mm.,  the  Upper  Part  showing  the  Appearance  produced  by  the  Silver 
Method  of  Demonstrating  the  Neuroglia  Fibers. 

a,  Ependyma  of  floor  plate;  b,  boundary  between  mantle  layer  and  marginal 
zone;  cs,  mesenchymal  connective- tissue  syncytium;  ep,  ependymal  cells;  i,  ingrowth 
of  connective  tissue;  m,  marginal  velum;  mn,  mantle  layer;  mv,  mantle  layer  of  floor 
plate;  p,  pia  mater;  r,  neuroglia  fibers. — (Hardesty.) 

The  latter  are  for  the"  most  part  small  and  are  scattered  among  the 
neuroblasts,  these,  on  the  other  hand,  being  larger  and  each  early 
developing  a  single  strong  process  which  grows  out  into  the  marginal 
velum  and  is  known  as  an  axis-cylinder.     At  a  later  period  the 


THE   HISTOGENESIS    OF    THE    NERVOUS    SYSTEM 


379 


neuroblasts  also  give  rise  to  other  processes,  termed  dendrites,  more 
slender  and  shorter  than  the  axis-cylinders,  branching  repeatedly, 
and,  as  a  rule,  not  extending  beyond  the  limits  of  the  mantle  layer. 
In  connection  with  the  neuroglia  cells  peculiar  neuroglia  fibrils 
develop  very  much  in  the  same  way  as  the  fibers  are  formed  in  mesen- 
chymal connective  tissue.  That  is  to  say,  they  are  formed  from  the 
peripheral  portions  of  the  cytoplasm  of  the  neuroglial  and  ependy- 
mal  cells.  But  since  these  cells  are  connectedi  together  to  form  a 
syncytium  the  fibrils  are  not  confined  to  the  territories  of  the  indi- 


o  ^i^r 

Ote%0«» 

OqQ    q  ®», 

■ooq.©^ 

urtOO^*>^  a 


Dooo§ 


o0o°b 

uouo  ° 


Fig.  228. — Diagrams  showing  the  Development  of  the  Mantle  Layer  in  the 

Spinal  Cord. 
The  circles,  indifferent  cells;  circles  with  dots,  neuroglia  cells;  shaded  cells,  germinal 
cells;  circles  with  cross,  germinal  cells  in  mitosis;  black  cells,  nerve-cells. — {Schaper.) 

vidual  cells,  but  may  extend  far  beyond  these,  passing  in  the  syncy- 
tium from  the  territory  of  one  neuroglial  cell  to  another,  many  of 
those,  indeed,  arising  in  connection  with  the  ependymal  cells  extend- 
ing throughout  the  entire  thickness  of  the  medullary  wall  (Fig.  227). 
The  fibrils  branch  abundantly  and  form  a  supportive  network 
extending  through  all  portions  of  the  central  nervous  system. 
The  axis-cylinder  processes  of  the  majority  of  the  neuroblasts  on 
reaching  the  marginal  velum  bend  upward  or  downward  and,  after 


38o 


THE   HISTOGENESIS    OF   THE    NERVOUS    SYSTEM 


traversing  a  greater  or  less  length  of  the  cord,  re-enter  the  mantle 
layer  and  terminate  by  dividing  into  numerous  short  branches  which 
come  into  relation  with  the  dendrites  of  adjacent  neuroblasts. 
The  processes  of  certain  cells  situated  in  the  ventral  region  of  the 
mantle  zone  pass,  however,  directly  through  the  marginal  velum 
out  into  the  surrounding  tissues  and  constitute  the  ventral  nerve- 
roots  (Fig.  231). 

The  dorsal  nerve-roots  have  a  very  different  origin.     In  embryos 

of  about  2.5  mm.,  in  which  the 
medullary  canal  is  only  partly 
closed  (Fig.  53),  the  cells  which 
lie  along  the  line  of  transition 
between  the  lips  of  the  groove 
and  the  general  ectoderm  form 
a  distinct  ridge  readily  recog- 
nized in  sections  and  termed  the 
neural  crest  (Fig.  229,  A).  When 
the  lips  of  the  groove  fuse  to- 
gether the  cells  of  the  crest  unite 
to  form  a  wedge-shaped  mass, 
completing  the  closure  of  the 
canal  (Fig.  229,  B),  and  later 
proliferate  so  as  to  extend  out- 
ward over  the  surface  of  the 
canal  (Fig.  229,  C).  Since  this 
proliferation  is  most  active  in  the 
regions  of  the  crest  which  corre- 
spond to  the  mesodermic  somites 
there  is  formed  a  series  of  cell  masses,  arranged  segmentally 
and  situated  in  the  mesenchyme  at  the  sides  of  the  medullary 
canal  (Fig.  214).  These  cell  masses  represent  the  dorsal  root 
ganglia,  and  certain  of  their  constituent  cells,  which  may  also  be 
termed  neuroblasts,  early  assume  a  fusiform  shape  and  send  out  a 
process  from  each  extremity.  One  of  these  processes,  the  axis- 
cylinder,  grows  inward  toward  the  medullary  canal  and  penetrates  its 


Fig.  229. — Three  Sections  through 
the  Medullary  Canal  of  an  Embryo 
of  2.5  mm. — (vonLenhossek.) 


THE   HISTOGENESIS    OF    THE    NERVOUS    SYSTEM  381 

marginal  velum,  and,  after  a  longer  or  shorter  course  in  this  zone, 
enters  the  mantle  layer  and  comes  into  contact  with  the  dendrites  of 
some  of  the  central  neuroblasts.  The  other  process  extends  per- 
ipherally and  is  to  be  regarded  as  an  extremely  elongated  dendrite. 
The  processes  from  the  cells  of  each  ganglion  aggregate  to  form  a 
nerve,  that  formed  by  the  axis-cylinders  being  the  posterior  root  of 
a  spinal  nerve,  while  that  formed  by  the  dendrites  soon  unites  with 
the  ventral  nerve-root  of  the  corresponding  segment  to  form  the 
main  stem  of  a  spinal  nerve. 

There  is  thus  a  very  important  difference  in  the  mode  of  develop- 
ment of  the  two  nerve-roots,  the  axis-cylinders  of  the  ventral  roots 


Fig.  230. — Cells  from  the  Gasserian  Ganglion  of  a  Guinea-pig  Embryo. 
a,  Bipolar  cell;  b  and  c,  transitional  stages  to  d,  T-shaped  cells. — (von  Gehuchten.) 

arising  from  cells  situated  in  the  wall  of  the  medullary  canal  and  grow- 
ing outward  (centrif ugally) ,  while  those  of  the  dorsal  root  spring 
from  cells  situated  peripherally  and  grow  inward  (centripetally) 
toward  the  medullary  canal.  In  the  majority  of  the  dorsal  root 
ganglia  the  points  of  origin  of  the  two  processes  of  each  bi-polar 
cell  gradually  approach  one  another  (Fig.  230,  b)  and  eventually 
come  to  rise  from  a  common  stem,  a  process  of  the  cell-body,  which 
thus  assumes  a  characteristic  T  form  (Fig.  230,  d). 

From  what  has  been  said  it  will  be  seen  that  each  axis-cylinder  is 
an  outgrowth  from  a  single  neuroblast  and  is  part  of  its  cell-body,  as  are 
also  the  dendrites.     Another  view  has,  however,  been  advanced  to  the 


382  THE    HISTOGENESIS    OF    THE    NERVOUS    SYSTEM 

effect  that  the  nerve  fibers  first  appear  as  chains  of  cells  and  that  the  axis- 
cylinders,  being  differentiated  from  the  cytoplasm  of  the  chains,  are  really 
multicellular  products.  Many  difficulties  stand  in  the  way  of  the  ac- 
ceptance of  this  view  and  recent  observations,  both  histogenetic  (Cajal) 
and  experimental  (Harrison),  tend  to  confirm  the  unicellular  origin  of 
the  axis-cylinders.  The  embryological  evidence  therefore  goes  to  support 
the  neurone  theory,  which  regards  the  entire  nervous  system  as  com- 
posed of  definite  units,  each  of  which  corresponds  to  a  single  cell  and  is 
termed  a  neurone. 

By  the  development  of  the  axis-cylinders  which  occupy  the  meshes 
of  the  marginal  velum,  that  zone  increases  in  thickness  and  comes 
to  consist  principally  of  nerve-fibers,  while  the  cell-bodies  of  the 
neurones  of  the  cord  are  situated  in  the  mantle  zone.  No  such  de- 
finite distinction  of  color  in  the  two  zones  as  exists  in  the  adult  is, 
however,  noticeable  until  a  late  period  of  development,  the  medullary 
sheaths,  which  give  to  the  nerve-fibers  their  white  appearance  not 
beginning  to  appear  until  the  fifth  month  and  continuing  to  form 
from  that  time  onward  until  after  birth.  The  origin  of  the  myelin 
which  composes  the  medullary  sheaths  is  as  yet  uncertain,  although 
the  more  recent  observations  tend  to  show  that  it  is  picked  out  from 
the  blood  and  deposited  around  the  axis-cylinders  in  some  manner 
not  yet  understood.  Its  appearance  is  of  importance  as  being 
associated  with  the  beginning  of  the  functional  activity  of  the 
nerve-fibers. 

In  addition  to  the  medullary  sheaths  the  majority  of  the  fibers 
of  the  peripheral  nervous  system  are  provided  with  primitive  sheaths, 
which  are  lacking,  however,  to  the  fibers  of  the  central  system. 
They  are  formed  by  cells  which  wander  out  from  the  dorsal 
root-ganglia  and  are  therefore  of  ectodermal  origin.  Frog  larvae 
deprived  of  their  neural  crests  at  an  early  stage  of  development 
produce  ventral  nerve-fibers  altogether  destitute  of  primitive 
sheaths  (Harrison). 

Various  theories  have  been  advanced  to  account  for  the  formation  of 
the  medullary  sheaths.  It  has  been  held  that  the  myelin  is  formed  at  the 
expense  of  the  outermost  portions  of  the  axis-cylinders  themselves  (von 
Kolliker),  and  on  the  other  hand,  it  has  been  regarded  as  an  excretion 
of  the  cells  which  compose  the  primitive  sheaths  surrounding  the  fibers 


THE    SPINAL   CORD  383 

(Ranvier) ,  a  theory  which  is,  however,  invalidated  by  the  fact  that  myelin  is 
formed  around  the  fibers  of  the  central  nervous  system  which  possess  no 
primitive  sheaths.  As  stated  above,  the  more  recent  observations 
(Wlassak)  indicate  its  exogenous  origin. 

It  has  been  seen  that  the  central  canal  is  closed  in  the  mid-dorsal 
line  by  a  mass  of  cells  derived  from  the  neural  crest.  These  cells 
do  not  take  part  in  the  formation  of  the  mantle  layer,  but  become 
completely  converted  into  ependymal  tissue,  and  the  same  is  true  of 
the  cells  situated  in  the  mid-ventral  line  of  the  canal.  In  these  two 
regions,  known  as  the  roof -plate  and  floor -plate  respectively,  the 
wall  of  the  canal  has  a  characteristic  structure  and  does  not  share 
to  any  great  extent  in  the  increase  of  thickness  which  distinguishes 
the  other  regions  (Fig.  231).  In  the  lateral  walls  of  the  canal  there 
is  also  noticeable  a  differentiation  into  two  regions,  a  dorsal  one 
standing  in  relation  to  the  ingrowing  fibers  from  the  dorsal  root 
ganglia  and  known  as  the  dorsal  zone,  and  a  ventral  one,  the  ventral 
zone,  similarly  related  to  the  ventral  nerve-roots.  In  different 
regions  of  the  medullary  tube  these  zones,  as  well  as  the  roof-  and 
floor-plates,  undergo  different  degrees  of  development,  producing 
peculiarities  which  may  now  be  considered. 

Trie  Development  of  the  Spinal  Cord. — Even  before  the  lips 
of  the  medullary  groove  have  met  a  marked  enlargement  of  the 
anterior  portion  of  the  canal  is  noticeable,  the  region  which  will 
become  the  brain  being  thus  distinguished  from  the  more  posterior 
portion  which  will  be  converted  into  the  spinal  cord.  When  the 
formation  of  the  mesodermic  somites  is  completed,  the  spinal  cord 
terminates  at  the  level  of  the  last  somite,  and  in  this  region  still 
retains  its  connection  with  the  ectoderm  of  the  dorsal  surface  of 
the  body;  but  in  that  portion  of  the  cord  which  is  posterior  to  the 
first  coccygeal  segment  the  histological  differentiation  does  not 
proceed  beyond  the  stage  when  the  walls  consist  of  several  layers  of 
similar  cells,  the  formation  of  neuroblasts  and  nerve-roots  ceasing 
with  the  segment  named.  After  the  fourth  month  the  more  differ- 
entiated portion  elongates  at  a  much  slower  rate  than  the  surround- 
ing tissues  and  so  appears  to  recede  up  the  spinal  canal,  until  its 


384  THE    SPINAL    CORD 

termination  is  opposite  the  second  lumber  vertebra.  The  less 
differentiated  portion,  which  retains  its  connection  with  the  ectoderm 
until  about  the  fifth  month,  is,  on  the  other  hand,  drawn  out  into  a 
slender  filament  whose  cells  degenerate  during  the  sixth  month, 
except  in  its  uppermost  part,  so  that  it  comes  to  be  represented 
throughout  the  greater  part  of  its  extent  by  a  thin  cord  composed 
of  pia  mater.  This  cord  is  the  structure  known  in  the  adult  as  the 
filum  terminate,  and  lies  in  the  center  of  a  leash  of  nerves  occupying 
the  lower  part  of  the  spinal  canal  and  termed  the  cauda  equina. 
The  existence  of  the  cauda  is  due  to  the  recession  of  the  cord  which 
necessitates  for  the  lower  lumbar,  sacral  and  coccygeal  nerves,  a 
descent  through  the  spinal  canal  for  a  greater  or  less  distance, 
before  they  can  reach  the  intervertebral  foramina  through  which 
they  make  their  exit. 

In  the  early  stages  of  development  the  central  canal  of  the  cord 
is  quite  large  and  of  an  elongated  oval  form,  but  later  it  becomes 
somewhat  rhomboidal  in  shape  (Fig.  231,  A),  the  lateral  angles 
marking  the  boundaries  between  the  dorsal  and  ventral  zones. 
As  development  proceeds  the  sides  of  the  canal  in  the  dorsal  region 
gradually  approach  one  another  and  eventually  fuse,  so  that  this 
portion  of  the  canal  becomes  obliterated  (Fig.  231,  B)  and  is  indi- 
cated by  the  dorsal  longitudinal  fissure  in  the  adult  cord,  the  central 
canal  of  which  corresponds  to  the  ventral  portion  only  of  the  embry- 
onic cavity.  While  this  process  has  been  going  on  both  the  roof- 
and  the  floor-plate  have  become  depressed  below  the  level  of  the 
general  surface  of  the  cord,  and  by  a  continuance  of  the  depression 
of  the  floor-plate — a  process  really  due  to  the  enlargement  and 
consequent  bulging  of  the  ventral  zone — the  anterior  median  fissure 
is  produced,  the  difference  between  its  shape  and  that  of  the  dorsal 
fissure  being  due  to  the  difference  in  its  development. 

The  development  of  the  mantle  layer  proceeds  at  first  more 
rapidly  in  the  ventral  zone  than  in  the  dorsal,  so  that  at  an  early 
stage  (Fig.  231,  A)  the  anterior  column  of  gray  matter  is  much  more 
pronounced,  but  on  the  development  of  the  dorsal  nerve-roots  the 
formation  of  neuroblasts  in  the  dorsal  zone  proceeds  apace,  resulting 


THE    SPINAL    CORD 


385 


in  the  formation  of  a  dorsal  column.  A  small  portion  of  the  zone, 
situated  between  the  point  of  entrance  of  the  dorsal  nerve-roots  and 
the  roof-plate,  fails,  however,  to  give  rise  to  neuroblasts  and  is 
entirely  converted  into  ependyma.  This  represents  the  future 
funiculus  gracilis  (fasciculus  of  Goll)  (Fig.  231,  A,  cG),  and  at  the 
point  of  entrance  of  the  dorsal  roots  into  the  cord  a  well-marked 
oval  bundle  of  fibers  is  formed  (Fig.  231,  A,  ob)  which,  as  develop- 


Fig.  231. — Transverse  Sections  through  the  Spinal  Cords  of  Embryos  .of  (A) 
about  Four  and  a  Half  Weeks  and  (B)  about  Three  Months'. 
cB,  Fasciculus  of  Burdach;  cG,  fasciculus  of  Goll;  dh,  dorsal  column;  dz,  dorsal 
zone;  fp,  floor-plate;  ob,  oval  bundle;  rp,  roof-plate;  vh,  ventral  column;  vz,  ventral  zone. 
— {His.) 

ment  proceeds,  creeps  dorsally  over  the  surface  of  the  dorsal  horn 
until  it  meets  the  lateral  surface  of  the  fasciculus  of  Goll,  and,  its 
further  progress  toward  the  median  line  being  thus  impeded,  it 
insinuates  itself  between  that  fasciculus  and  the  posterior  horn  to 
form  the  funiculus  cuneatus  {fasciculus  of  Burdach)  (Fig.  231,  B,  cB). 

Little  definite  is  as  yet  known  concerning  the  development  of  the 
other  fasciculi  which  are  recognizable  in  the  adult  cord,  but  it  seems 

25 


386 


THE    BRAIN 


A    " 

; 

t 

ffe 

V/tt 

1  — H 

/»// 

\-mt 

certain  that  the  lateral  and  anterior  cerebro-spinal  (pyramidal)  fasciculi 
are  composed  of  fibers  which  grow  downward  in  the  meshes  of  the 
marginal  velum  from  neuroblasts  situated  in  the  cerebral  cortex,  while 
the  cerebellospinal  (direct  cerebellar)  fasciculi  and  the  fibers  of  the 
ground-bundles  have  their  origin  from  cells  of  the  mantle  layer  of  the 
cord. 

The  myelination  of  the  fibers  of  the  spinal  cord  begins  between  the 
fifth  and  sixth  months  and  appears  first  in  the  funiculi  cuneati,  and  about 

a  month  later  in  the  funiculi  graciles. 
The  myelination  of  the  great  motor  paths, 
the  lateral  and  anterior  cerebro-spinal  fas- 
ciculi, is  the  last  to  develop,  appearing  to- 
ward the  end  of  the  ninth  month  of  fetal 
life. 

The  Development  of  the  Brain. 

— The  enlargement  of  the  anterior 
portion  of  the  medullary  canal  does 
not  take  place  quite  uniformly,  but  is 
less  along  two  transverse  lines  than  else 
where,  so  that  the  brain  region  early 
becomes  divided  into  three  primary 
vesicles  which  undergo  further  differ- 
entiation as  follows.  Upon  each  side 
of  the  anterior  vesicle  an  evagination 
appears  and  becomes  converted  into  a 
club-shaped  structure  attached  to  the 
ventral  portion  of  the  vesicle  by  a 
pedicle.  These  evaginations  (Fig. 
232,  op)  are  known  as  the  optic  evag- 
inations, and  being  concerned  in  the 
formation  of  the  eye  will  be  considered 
in  the  succeeding  chapter.  After  their 
formation  the  antero-lateral  portions 
of  the  vesicle  become  bulged  out  into  two  protuberances  (h)  which 
rapidly  increase  in  size  and  give  rise,  eventually  to  the  two  cerebral 
hemispheres,  which  form,  together  with  the  portion  of  the  vesicle 
which  lies  between  them,  what  is  termed  the  telencephalon  or  fore- 
brain,  the  remainder  of  the  vesicle  giving  rise  to  what  is  known  as 


Fig.  232. — Reconstruction  of 
the  Brain  of  an  Embryo  of  2.15 

MM. 

h,  Hemisphere;  i,  isthmus;  m, 
mesencephalon;  mf,  mid-brain  flex- 
ure; mt,  metencephalon ;  myl,  myel- 
encephalon;  nf,  nape  flexure;  ot,  otic 
capsule;  op,  optic  evagination;  t, 
diencephalon. — (His.) 


THE    BRAIN 


387 


the  diencephalon  or  Hween-brain  (Fig.  232,  /).  The  middle  vesicle  is 
bodily  converted  into  the  mesencephalon  or  mid-brain  (m),  but  the 
posterior  vesicle  differentiates  so  that  three  parts  may  be  recognized : 
(1)  a  rather  narrow  portion  which  immediately  succeeds  the  mid- 
brain and  is  termed  the  isthmus  (i);  (2)  a  portion  whose  roof  and 
floor  give  rise  to  the  cerebellum  and  pons  respectively,  and  which  is 
termed  the  metencephalon  or  hind-brain  (mi) ;  and  (3)  a  terminal  por- 
tion which  is  known  as  the  medulla  oblongata,  or,  to  retain  a  con- 
sistent nomenclature,  the  myelencephalon  or  after-brain  {my).  From 
each  of  these  six  divisions  definite  structures  arise  whose  relations 
to  the  secondary  divisions  and  to  the  primary  vesicles  may  be  un- 
derstood from  the  following  table  and  from  the  annexed  figure  (Fig. 
233),  which  represents  a  median  longitudinal  section  of  the  brain 
of  a  fetus  of  three  months. 


3rd  Vesicle 


Myelencephalon 


Metencephalon 


Isthmus 


Medulla  oblongata  (I) . 

/  Pons  (II  1). 

^  Cerebellum  (II  2). 

SBrachia  conjunctiva  (III). 
Cerebral  peduncles  (posterior 
portion) . 


2nd  Vesicle Mesencephalon 


Cerebral   peduncles    (anterior  por- 
tion) (IV  1). 
Corpora  quadrigemina  (IV  2). 


1st  Vesicle < 


Diencephalon 


Telencephalon 


Pars  mammillaris  (V  1). 
Thalamus  (V  2). 
Epiphysis  (V  3). 

Infundibulum  (VI  1). 
Corpus  striatum  (VI  2). 
Olfactory  bulb  (VI  3). 
Hemispheres  (VI  4). 


But  while  the  walls  of  the  primary  vesicles  undergo  this  complex 
differentiation,  their  cavities  retain  much  more  perfectly  their 
original  relations,  only  that  of  the  first  vesicle  sharing  to  any  great 
extent  the  modifications  of  the  walls. 


388  THE    BRAIN 

The  cavity  of  the  third  vesicle  persists  in  the  adult  as  the  fourth 
ventricle,  traversing  all  the  subdivisions  of  the  vesicle;  that  of  the 
second,  increasing  but  little  in  height  and  breadth,  constitutes  the 
aquaductus  cerebri;  while  that  of  the  first  vesicle  is  continued  into 
the  cerebral  hemispheres  to  form  the  lateral  ventricles,  the  remainder 
of  it  constituting  the  third  ventricle,  which  includes  the  cavity  of 
the  median  portion  of  the  telencephalon  as  well  as  the  entire  cavity 
of  the  diencephalon. 

During  the  differentiation  of  the  various  divisions  of  the  brain 
certain  flexures  appear  in  the  roof  and  floor,  and  to  a  certain  extent 


'V'i  L-/ 


IV  Z 


iVi 


02 


Fig.  233. — Median  Longitudinal  Section  of  the  Brain  of  an  Embryo  of  the 
Third  Month. — (His.) 

correspond  with  those  already  described  as  occurring  in  the  embryo. 
The  first  of  these  flexures  to  appear  occurs  in  the  region  of  the  mid- 
brain, the  first  vesicle  being  bent  ventrally  until  it  comes  to  lie  at 
practically  a  right  angle  with  the  axis  of  the  mid-brain.  This  may 
be  termed  the  mid-brain  flexure  (Fig.  232,  mf)  and  corresponds  with 
the  head-bend  of  the  embryo.  The  second  flexure  occurs  in  the 
region  of  the  medulla  oblongata  and  is  known  as  the  nape  flexure 
(Fig.  232,  nf);  it  corresponds  with  the  similarly  named  bend  of  the 
embryo  and  is  produced  by  a  bending  ventrally  of  the  entire  head,  so 


THE     MYELENCEPHALON  389 

that  the  axis  of  the  mid-brain  comes  to  lie  almost  at  right  angles 
with  that  of  the  medulla  and  that  of  the  first  vesicle  parallel  with  it. 
Finally,  a  third  flexure  occurs  in  the  region  of  the  metencephalon 
and  is  entirely  peculiar  to  the  nervous  system;  it  consists  of  a  bending 
ventrally  of  the  floor  of  the  hind-brain,  the  roof  of  this  portion  of  the 
brain  not  being  affected  by  it,  and  it  may  consequently  be  known  as 
the  pons  flexure  (Fig.  233). 

In  the  later  development  the  pons  flexure  practically  disappears, 
owing  to  the  development  in  this  region  of  the  transverse  fibers  and 
nuclei  of  the  pons,  but  the  mid-brain  and  nape  flexures  persist, 
though  greatly  reduced  in  acuteness,  the  axis  of  the  anterior  portion 
of  the  adult  brain  being  inclined  to  that  of  the  medulla  at  an  angle  of 
about  134  degrees. 

The  Development  of  the  Myelencephalon. — In  its  posterior  portion 
the  myelencephalon  closely  resembles  the  spinal  cord  and  has  a  very 
similar  development.  More  anteriorly,  however,  the  roof-plate 
(Fig.  234,  rp)  widens  to  form  an  exceedingly  thin  membrane,  the 
posterior  velum;  with  the  broadening  of  the  roof-plate  there  is  asso- 
ciated a  broadening  of  the  dorsal  portion  of  the  brain  cavity,  the 
dorsal  and  ventral  zones  bending  outward,  until,  in  the  anterior 
portion  of  the  after-brain,  the  margins  of  the  dorsal  zone  have  a 
lateral  position,  and  are,  indeed,  bent  ventrally  to  form  a  reflected 
lip  (Fig.  234,  I).  The  portion  of  the  fourth  ventricle  contained  in 
this  division  of  the  brain  becomes  thus  converted  into  a  broad  shallow 
cavity,  whose  floor  is  formed  by  the  ventral  zones  separated  in  the 
median  line  by  a  deep  groove,  the  floor  of  which  is  the  somewhat 
thickened  floor-plate.  About  the  fourth  month  there  appears  in  the 
roof-plate  a  transverse  groove  into  which  the  surrounding  mesen- 
chyme dips,  and,  as  the  groove  deepens  in  later  stages,  the  mesen- 
chyme contained  within  it  becomes  converted  into  blood-vessels, 
forming  the  chorioid  plexus  of  the  fourth  ventricle,  a  structure  which, 
as  may  be  seen  from  its  development,  does  not  lie  within  the  cavity 
of  the  ventricle,  but  is  separated  from  it  by  the  portion  of  the  roof- 
plate  which  forms  the  floor  of  the  groove. 

In  embryos  of  about  9  mm.  the  differentiation  of  the  dorsal 


39o 


THE    MYELENCEPHALON 


and  ventral  zones  into  ependymal  and  mantle  layers  is  clearly  visible 
(Fig.  234),  and  in  the  ventral  zone  the  marginal  velum  is  also  well 
developed.  Where  the  fibers  from  the  sensory  ganglion  of  the  vagus 
nerve  enter  the  dorsal  zone  an  oval  area  (Fig.  234,  fs)  is  to  be  seen 
which  is  evidently  comparable  to  the  oval  bundle  of  the  cord  and 
consequently  with  the  fasciculus  of  Burdach.  It  gives  rise  to  the 
solitary  fasciculus  of  adult  anatomy,  and  in  embryos  of  11  to  13  mm. 
it  becomes  covered  in  by  the  fusion  of  the  reflected  lip  of  the  dorsal 
zone  with  the  sides  of  the  myelencephalon,  this  fusion,  at  the  same 
time,  drawing  the  margins  of  the  roof-plate  ventrally  to  form  a 


Fig.  234. — Transverse  Section  through  the  Medulla  Oblongata  of 

an  Embryo  of  9.1  mm. 

dz,  Dorsal  zone;  fp,  floor-plate; /s,  fasciculus  solitarius;  I,  lip;  rp,  roof-plate;  vz,  ventral 

zone;  X  and  XII,  tenth  and  twelfth  nerves. —  (His.) 

secondary  lip  (Fig.  235).  Soon  after  this  a  remarkable  migration 
ventrally  of  neuroblasts  of  the  dorsal  zone  begins.  Increasing 
rapidly  in  number  the  migrating  cells  pass  on  either  side  of  the  soli- 
tary fasciculus  toward  the  territory  of  the  ventral  zone,  and,  passing 
ventrally  to  the  ventral  portion  of  the  mantle  layer,  into  which 
fibers  have  penetrated  and  which  becomes  the  formatio  reticularis 
(Fig.  235,  fr),  they  differentiate  to  form  the  olivary  body  (ol). 

The  thickening  of  the  floor-plate  gives  opportunity  for  fibers  to 
pass  across  the  median  line  from  one  side  to  the  other,  and  this 
opportunity  is  taken  advantage  of  at  an  early  stage  by  the  axis-cylin- 


THE    MYELENCEPHALON 


391 


ders  of  the  neuroblasts  of  the  ventral  zone,  and  later,  on  the  establish- 
ment of  the  olivary  bodies,  other  fibers,  descending  from  the  cere- 
bellum, decussate  in  this  region  to  pass  to  the  olivary  body  of  the 
opposite  side.  In  the  lower  part  of  the  medulla  fibers  from  the 
neuroblasts  of  the  nuclei  gracilis  and  cuneatus,  which  seem  to  be 


ol  z* 

Fig.  235. — Transverse  Section  through  the  Medulla  Oblongata  of  an  Embryo 

of  about  Eight  Weeks. 

av,  Ascending  root  of  the  trigeminus ;fr,  reticular  formation;  ol,  olivary  body;  sf,  solitary 

fasciculus;  tr,  restiform  body;  XII,  hypoglossal  nerve. — (His.) 

developments  from  the  mantle  layer  of  the  dorsal  zone,  also  decussate 
in  the  substance  of  the  floor-plate;  these  fibers,  known  as  the  arcuate 
fibers,  pass  in  part  to  the  cerebellum,  associating  themselves  with 
fibers  ascending  from  the  spinal  cord  and  with  the  olivary  fibers  to 
form  a  round  bundle  situated  in  the  dorsal  portion  of  the  marginal 
velum  and  known  as  the  restiform  body  (Fig.  235,  tr). 

The  principal  differentiations  of  the  zones  of  the  myelencephalon 
may  be  stated  in  tabular  form  as  follows: 

Roof-plate Posterior  velum. 

(Nuclei  of  termination  of  sensory  roots  of  cranial  nerves. 
Nuclei  gracilis  and  cuneatus. 
The  olivary  bodies. 

.  (  Nuclei  of  origin  of  the  motor  roots  of  cranial  nerves. 

Ventral   zones <  _,,  ... 

I    I  he  reticular  formation. 

Foor-plate The  median  raphe. 


392 


THE    CEREBELLUM 


The  Development  of  the  Metencephalon  and  Isthmus. — Our  knowl- 
edge of  the  development  of  the  metencephalon,  isthmus,  and  mesen- 
cephalon is  by  no  means  as  complete  as  is  that  of  the  myelencephalon. 
The  pons  develops  as  a  thickening  of  the  portion  of  the  brain  floor 
which  forms  the  anterior  wall  of  the  pons  flexure,  and  its  transverse 
fibers  are  well  developed  by  the  fourth  month  (Mihalkovicz),  but  all 
details  regarding  the  origin  of  the  pons  nuclei  are  as  yet  wanting. 
If  one  may  argue  from  what  occurs  in  the  myelencephalon,  it  seems 
probable  that  the  reticular  formation  of  the  metencephalon  is  derived 
from  the  ventral  zone,  and  that  the  median  raphe  represents  the 
floor-plate.  Furthermore,  the  relations  of  the  pons  nuclei  to  the 
reticular  formation  on  the  one  hand,  and  its  connection  by  means  of 


Fig.  236. — A,  Dorsal  View  of  the  Brain  or  a  Rabbit  Embryo  of  16  mm.;  B,  Median 

Longitudinal  Section  of  a  Calf  Embryo  of  3  cm. 

c,  Cerebellum;  m,  mid-brain. — {Mihalkovicz?) 

the  transverse  pons  fibers  with  the  cerebellum  on  the  other,  suggest 
the  possibility  that  they  may  be  the  metencephalic  representatives 
of  the  olivary  bodies  and  are  formed  by  a  migration  ventrally  of 
neuroblasts  from  the  dorsal  zones,  such  a  migration  having  been 
observed  to  occur  (Essick). 

The  cerebellum  is  formed  from  the  dorsal  zones  and  roof-plate 
of  the  metencephalon  and  is  a  thickening  of  the  tissue  immediately 
anterior  to  the  front  edge  of  the  posterior  velum.  This  latter  struc- 
ture has  in  early  stages  a  rhomboidal  shape  (Fig.  236,  A)  which 
causes  the  cerebellar  thickening  to  appear  at  first  as  if  composed 
of  two  lateral  portions  inclined  obliquely  toward  one  another.  In 
reality,  however,  the  thickening  extends  entirely  across  the  roof  of 


THE    CEREBELLUM 


393 


the  brain  (Fig.  236,  B),  the  roof-plate  probably  being  invaded  by 
cells  from  the  dorsal  zones  and  so  giving  rise  to  the  vermis,  while  the 
lobes  are  formed  directly  from  the  dorsal  zones.  During  the  second 
month  a  groove  appears  on  the  ventral  surface  of  each  lobe,  marking 
out  an  area  which  becomes  the  flocculus,  and  later,  during  the  third 
month,  transverse  furrows  appear  upon  the  vermis  dividing  it  into 
five  lobes,  and  later  still  extend  out  upon  the  lobes  and  increase  in 
number  to  produce  the  lamel- 
late structure  characteristic  of 
the  cerebellum. 

The  histogenetic  develop- 
ment of  the  cerebellum  at  first 
proceeds  along  the  lines  which 
have  already  been  described 
as  typical,  but  after  the  devel- 
opment of  the  mantle  layer  the 
cells  lining  the  greater  portion 
of  the  cavity  of  the  ventricle 

rease  to  rrmltinlv  onlv  those  FlG-  237-— Diagram  Representing  the 
cease  to  multiply,  oniy   tnose    DifferenTiation  of  the  Cerebellar  Cells. 

which  are  situated  in  the  roof-        The  circles,   indifferent  cells;   circles   with 

plate    of    the   metencephalon    d°f '  n<rur.0glia  cfs>  shaded  c?lls:  g™.al 

1  r  cells;    circles   with   cross,   germinal    cells    in 

and  along  the  line  of  junction     mitosis;    black    cells,  nerve-cells.  L,  Lateral 

.      ,  ,     ,,         ,i  •  i        •  recess;  M,  median  furrow,  and  R,  floor  of  IV, 

of  the   cerebellar   thickening    fourth  ventricle.— (Schaper.) 
with  the  roof-plate  continuing 

to  divide.  The  indifferent  cells  formed  in  these  regions  migrate 
outward  from  the  median  line  and  forward  in  the  marginal  ve- 
lum to  form  a  superficial  layer,  known  as  the  epithelioid  layer, 
and  cover  the  entire  surface  of  the  cerebellum  (Fig.  237).  The 
cells  of  this  layer,  like  those  of  the  mantle,  differentiate  into  neuroglia 
cells  and  neuroblasts,  the  latter  for  the  most  part  migrating  centrally 
at  a  later  stage  to  mingle  with  the  cells  of  the  mantle  layer  and  to 
become  transformed  into  the  granular  cells  of  the  cerebellar  cortex. 
The  neuroglia  cells  remain  at  the  surface,  however,  forming  the 
principal  constituent  of  the  outer  or,  as  it  is  now  termed,  the  molecular 
layer  of  the  cortex,  and  into  this  the  dendrites  of  the  Purkinje  cells, 


394  THE  isthmus 

probably  derived  from  the  mantle  layer,  project.  The  migration 
of  the  neuroblasts  of  the  epithelial  layer  is  probably  completed 
before  birth,  at  which  time  but  few  remain  in  the  molecular  layer 
to  form  the  stellate  cells  of  the  adult.  The  origin  of  the  dentate  and 
other  nuclei  of  the  cerebellum  is  at  present  unknown,  but  it  seems 
probable  that  they  arise  from  cells  of  the  mantle  layer. 

The  nerve-fibers  which  form  the  medullary  substance  of  the 
cerebellum  do  not  make  their  appearance  until  about  the  sixth 
month,  when  they  are  to  be  found  in  the  ependymal  tissue  on  the 
inner  surface  of  the  layer  of  granular  cells.  Those  which  are  not 
commissural  or  associative  in  function  converge  to  the  line  of  junction 
of  the  cerebellum  with  the  pons,  and  there  pass  into  the  marginal 
velum  of  the  pons,  myelencephalon,  or  isthmus  as  the  case  may  be. 

The  dorsal  surface  of  the  isthmus  is  at  first  barely  distinguishable 
from  the  cerebellum,  but  as  development  proceeds  its  roof-plate 
undergoes  changes  similar  to  those  occurring  in  the  medulla  ob- 
longata and  becomes  converted  into  the  anterior  velum.  In  the 
dorsal  portion  of  its  marginal  velum  fibers  passing  to  and  from  the 
cerebellum  appear  and  form  the  brachia  conjunctiva,  while  ventrally 
fibers,  descending  from  the  more  anterior  portions  of  the  brain,  form 
the  cerebral  peduncles.  Nothing  is  at  present  known  as  to  the  history 
of  the  gray  matter  of  this  division  of  the  brain,  although  it  may  be 
presumed  that  its  ventral  zones  take  part  in  the  formation  of  the 
tegmentum,  while  from  its  dorsal  zones  the  nuclei  of  the  brachia  con- 
junctiva are  possibly  derived. 

The  following  table  gives  the  origin  of  the  principal  structures  of 
the  metencephalon  and  isthmus: 

Metencephalon.  Isthmus. 

/  Posterior  velum.  Anterior  velum. 

^  Vermis  of  cerebellum. 


Dorsal  zones. 


Lobes  of  cerebellum.  Brachia  conjunctiva. 

Flocculi. 

Nuclei  of  termination  of  sen- 
sory roots  of  cranial  nerves. 
Pons  nuclei. 


THE    MESENCEPHALON  395 

Metencephalon.  Isthmus. 

f  Nuclei  of  origin  of  motor  Posterior     part     of     cerebral 

Ventral  zones -j       roots  of  cranial  nerves.  peduncles. 

[  Reticular  formation.  Posterior  part  of  tegmentum. 

Floor-plate Median  raphe.  Median  raphe. 

The  Development  of  the  Mesencephalon. — Our  knowledge  of  the 
development  of  this  portion  of  the  brain  is  again  very  imperfect. 
During  the  stages  when  the  flexures  of  the  brain  are  well  marked 
(Figs.  232  and  233)  it  forms  a  very  prominent  structure  and  pos- 
sesses for  a  time  a  capacious  cavity.  Later,  however,  it  increases  in 
size  less  rapidly  than  adjacent  parts  and  its  walls  thicken,  the  roof- 
and  floor-plates  as  well  as  the  zones,  and,  as  a  result,  the  cavity 
becomes  the  relatively  smaller  canal-like  cerebral  aquaeduct.  In  the 
marginal  velum  of  its  ventral  zone  fibers  appear  at  about  the  third 
month,  forming  the  anterior  portion  of  the  cerebral  peduncles,  and, 
at  the  same  time,  a  median  longitudinal  furrow  appears  upon  the 
dorsal  surface,  dividing  it  into  two  lateral  elevations  which,  in  the 
fifth  month,  are  divided  transversely  by  a  second  furrow  and  are 
thus  converted  from  corpora  bigemina  (in  which  form  they  are 
found  in  the  lower  vertebrates)  into  corpora  quadrigemina. 

Nothing  is  known  as  to  the  differentiation  of  the  gray  matter  of  the 
dorsal  and  ventral  zones  of  the  mid-brain.  From  the  relation  of  the  parts 
in  the  adult  it  seems  probable  that  in  addition  to  the  nuclei  of  origin  of 
the  oculomotor  and  trochlear  nerves,  the  ventral  zones  give  origin  to  the 
gray  matter  of  the  tegmentum,  which  is  the  forward  continuation  of  the 
reticular  formation.  Similarly  it  may  be  supposed  that  the  corpora 
quadrigemina  are  developments  of  the  dorsal  zones,  as  may  also  be  the 
red  nuclei,  whose  relations  to  the  brachia  conjunctiva  suggest  a  com- 
parison with  the  olivary  bodies  and  the  nuclei  of  the  pons. 

A  tentative  scheme  representing  the  origin  of  the  mid-brain  structures 
may  be  stated  thus: 

Roof -plate (?) 

J  Corpora  quadrigemina. 
.LJorsal  zones. ......  \  . 

^  Red  nuclei. 

[  Nuclei  of  origin  of  the  third  and  fourth  nerves. 
Ventral  zones \  Anterior  part  of  tegmentum. 

[  Anterior  part  of  cerebral  peduncles. 
Floor-plate Median  raphe. 


396 


THE   DIENCEPHALON 


The  Development  of  the  Diencephalon. — A  transverse  section 
through  the  diencephalon  of  an  embryo  of  about  five  weeks  (Fig. 
238)  shows  clearly  the  differentiation  of  this  portion  of  the  brain  into 
the  typical  zones,  the  roof-plate  {rp)  being  represented  by  a  thin- 
walled,  somewhat  folded  area,  the  floor-plate  (fp)  by  the  tissue 
forming  the  floor  of  a  well-marked  ventral  groove,  while  each  lateral 
wall  is  divided  into  a  dorsal  and  ventral  zone  by  a  groove  known  as 
the   sulcus   Monroi    (Sm),    which   extends   forward   and   ventrally 

toward  the  point  of  origin  of  the  optic 
evagination  (Fig.  240).  At  the  pos- 
terior end  of  the  ridge-like  elevation 
which  represents  the  roof-plate  is  a 
rounded  elevation  (Fig.  239,  p)  which, 
in  later  stages,  elongates  until  it  al- 
most reaches  the  dermis,  forming  a 
hollow  evagination  of  the  brain  roof 
known  as  the  pineal  process.  The  dis- 
tal extremity  of  this  process  enlarges  to 
a  sac-like  structure  which  later  be- 
Fig.  238.— Transverse  Section    comes  lobed,  and,  by  an  active  pro- 

of  the  Diencephalon  of  an  Em-    Hferation  of  the  cells  lining  the  cavi- 
bryo  of  Five  Weeks.  ^ 

dz,  Dorsal  zone;  fp,  floor-plate;     tieS    °f    the    various    lobes,    finally    be- 


rp,   roof-plate;  Sm,  sulcus  Monroi 
vz,  ventral  zone. —  (His.) 


comes  a  solid  structure,  the  pineal  body. 

The  more  proximal  portion  of  the 
evagination,  remaining  hollow,  forms  the  pineal  stalk,  and  the  en- 
tire structure,  body  and  stalk,  constitutes  what  is  known  as  the 
epiphysis. 

The  significance  of  this  organ  in  the  Mammalia  is  doubtful.  In  the 
Reptilia  and  other  lower  forms  the  outgrowth  is  double,  a  secondary 
outgrowth  arising  from  the  base  or  from  the  anterior  wall  of  the  primary 
one.  This  anterior  evagination  elongates  until  it  reaches  the  dorsal 
epidermis  of  the  head,  and,  there  expanding,  develops  into  an  unpaired 
eye,  the  epidermis  which  overlies  it  becoming  converted  into  a  trans- 
parent cornea.  In  the  Mammalia  this  anterior  process  does  not  develop 
and  the  epiphysis  in  these  forms  is  comparable  only  to  the  posterior 
process  of  the  Reptilia. 

In  addition  to  the  epiphysial  evaginations,  another  evagination  arises 


THE    DIENCEPHALON 


397 


from  the  roof-plate  of  the  first  brain  vesicle,  further  forward,  in  the  region 
which  becomes  the  median  portion  of  the  telencephalon.     This  paraphysis 
as  it  has  been  called,  has  been  observed  in  the  lower  vertebrates  and  in  the 
Marsupials  (Selenka),  but  up  to  the 
present  has  not  been  found  in  other 
groups  of  the  Mammalia.     It  seems  to 
be  comparable  to  a  chorioid  plexus 
which  is  evaginated  from  the  brain 
surface  instead  of  being  invaginated 
as  is  usually  the  case.     There  is  no  evi- 
dence that  a  paraphysis  is  developed 
in  the  human  brain. 

The  portion  of  the  roof-plate 
which  lies  in  front  of  the  epiphysis 
represents  the  velum  interpositum 
of  the  adult  brain,  and  it  forms  at 
first  a  distinct  ridge  (Fig.  239,  rp). 
At  an  early  stage,  however,  it  be- 
comes reduced  to  a  thin  membrane 
upon  the  surface  of  which  blood- 
vessels, developing  in  the  surround- 
ing mesenchyme,  arrange  them- 
selves at  about  the  third  month  in 
two  longitudinal  plexuses,  which, 
with  the  subjacent  portions  of  the 

velum,  become  invaginated  into  the       „  „, 

0  riG.    239. — Dorsal  View   of  the 

cavity  of  the  third  ventricle  to  form    Brain,   the  Roof  of  the  Lateral 

its  chorioid  Mexu*  Ventricles  being  Removed,   of  an 

us  cnomoia  plexus.  Embryo  of  13.6  mm. 

The     dorsal    zones    thicken    in         b,    Superior    brachiuui;    eg,    lateral 

their     more     dorsal     and     anterior    Seniculate.  body;  CP>  chorioid  plexus; 
tneir    more    aorsai    ana    anterior    cqa>  anterior  corpu3  quadrigeminum; 

portions  to  form  massive  Structures,     h>  hippocampus;  hf,  hippocampal  fis- 
,  7     7        .  ,     sure;  ot,  thalamus;  p,  pineal  body;  rp, 

the   thatami    [rigs.    233,    V2,    and    roof-plate.— (Aw.) 

239,  ot),  which,  encroaching  upon 

the  cavity  of  the  ventricle,    transform  it  into   a  narrow  slit-like 

space,  so  narrow,  indeed,  that  at  about  the  fifth  month  the  inner 

surfaces  of  the  two  thalami  come  in  contact  in  the  median  line, 

forming  what  is  known  as  the  intermediate  mass.     More  ventrally 


/,   ., 


-\P 


cqn 


398  THE    TELENCEPHALON 

and  posteriorly  another  thickening  of  the  dorsal  zone  occurs,  giving 
rise  on  each  side  to  the  pulvinar  of  the  thalamus  and  to  a  lateral 
geniculate  body,  and  two  ridges  extending  backward  and  dorsally 
from  the  latter  structures  to  the  thickenings  in  the  roof  of  the  mid- 
brain which  represent  the  anterior  corpora  quadrigemina,  give  a 
path  along  which  the  nerve-fibers  which  constitute  the  superior 
quadrigeminal  brachia  pass. 

From  the  ventral  zones  what  is  known  as  the  hypothalamic  region 
develops,  a  mass  of  fibers  and  cells  whose  relations  and  development 
are  not  yet  clearly  understood,  but  which  may  be  regarded  as  the 
forward  continuation  of  the  tegmentum  and  reticular  formation. 
In  the  median  line  of  the  floor  of  the  ventricle  an  unpaired  thickening 
appears,  representing  the  corpora  mamillaria,  which  during  the 
third  month  becomes  divided  by  a  median  furrow  into  two  rounded 
eminences;  but  whether  these  structures  and  the  posterior  portion 
of  the  tuber  cinereum,  which  also  develops  from  this  region  of  the 
brain,  are.  derivatives  of  the  ventral  zones  or  of  the  floor-plate  is  as 
yet  uncertain. 

Assuming  that  the  mamillaria  and  the  tuber  cinereum  are  derived 
from  the  ventral  zones,  the  origins  of  the  structures  formed  from  the 
walls  of  the  diencephalon  may  be  tabulated  as  follows: 

^      .    ,  f  Velum  interpositum. 

Roof-plate <   ^   .  .      .      ^ 

(_  Epiphysis. 

{Thalami. 
Pulvinares. 
Lateral  geniculate  bodies. 
{Hypothalamic  region. 
Corpora  mamillaria. 
Tuber  cinereum  (in  part) . 
Floor-plate Tissue  of  mid-ventral  line. 

The  Development  of  the  Telencephalon.- — For  convenience  of 
description  the  telencephalon  may  be  regarded  as  consisting  of  a 
median  portion,  which  contains  the  anterior  part  of  the  third  ven- 
tricle, and  two  lateral  outgrowths  which  constitute  the  cerebral 
hemispheres.  The  roof  of  the  median  portion  undergoes  the  same 
transformation  as  does  the  greater  portion  of  that  of  the  diencephalon 


THE    TELENCEPHALON  399 

and  is  converted  into  the  anterior  part  of  the  velum  interpositum 
(Fig.  240,  vi).  Anteriorly  this  passes  into  the  anterior  wall  of  the 
third  ventricle,  the  lamina  terminalis  {It),  a  structure  which  is  to  be 
regarded  as  formed  by  the  union  of  the  dorsal  zones  of  opposite 
sides,  since  it  lies  entirely  dorsal  to  the  anterior  end  of  the  sulcus 
Monroi.  From  the  ventral  part  of  the  dorsal  zones  the  optic 
evaginations  are  formed,  a  depression,  the  optic  recess  (or),  marking 
their  point  of  origin. 

The  ventral  zones  are  but  feebly  developed,  and  form  the  anterior 
part  of  the  hypothalamic  region,  while  at  the  anterior  extremity 
of  the  floor-plate  an  evagination  occurs,  the  infundibular  recess  (ir), 
which  elongates  to  form  a  funnel-shaped  structure  known  as  the 
hypophysis.  At  its  extremity  the  hypophysis  comes  in  contact 
during  the  fifth  week  with  the  enlarged  extremity  of  Rathke's  pouch 
formed  by  an  invagination  of  the  roof  of  the  oral  sinus  (see  p.  285), 
and  applies  itself  closely  to  the  posterior  surface  of  this  (Fig.  233) 
to  form  with  it  the  pituitary  body.  The  anterior  lobe  at  an  early 
stage  separates  from  the  mucous  membrane  of  the  oral  sinus,  the 
stalk  by  which  it  was  attached  completely  disappearing,  and  toward 
the  end  of  the  second  month  it  begins  to  send  out  processes  from 
its  walls  into  the  surrounding  mesenchyme  and  so  becomes  con- 
verted into  a  mass  of  solid  epithelial  cords  embedded  in  a  mesen- 
chyme rich  in  blood  and  lymphatic  vessels.  The  cords  later  on 
divide  transversely  to  a  greater  or  less  extent  to  form  alveoli,  the 
entire  structure  coming  to  resemble  somewhat  the  parathyreoid 
bodies  (see  p.  297),  and,  like  these,  having  the  function  of  producing 
an  internal  secretion.  The  posterior  lobe,  derived  from  the  brain, 
retains  its  connection  with  that  structure,  its  stalk  being  the  injun- 
dibidum,  but  its  terminal  portion  does  not  undergo  such  extensive 
modifications  as  does  the  anterior  lobe,  although  it  is  claimed  that 
it  gives  rise  to  a  glandular  epithelium  which  may  become  arranged 
so  as  to  form  alveoli. 

The  cerebral  hemispheres  are  formed  from  the  lateral  portions 
of  the  dorsal  zones,  each  possessing  also  a  prolongation  of  the  roof- 
plate.     From  the  more  ventral  portion  of  each  dorsal  zone  there  is 


400  THE    TELENCEPHALON 

formed  a  thickening,  the  corpus  striatum  (Figs.  240,  cs,  and  233,  VI  2), 
a  structure  which  is  for  the  telencephalon  what  the  optic  thalamus 
is  for  the  diencephalon,  while  from  the  more  dorsal  portion  there  is 
formed  the  remaining  or  mantle  {pallial)  portions  of  the  hemispheres 
(Figs.  240,  h,  and  233,  VI  4).  When  first  formed,  the  hemispheres 
are  slight  evaginations  from  the  median  portion  of  the  telencephalon, 
the  openings  by  which  their  cavities  communicate  with  the  third 
ventricle,  the  interventricular  foramina,  being  relatively  very  large 
(Fig.  240),  but,  in  later  stages  (Fig.  233),  the  hemispheres  increase 
more  markedly  and  eventually  surpass  all  the  other  portions  of  the 


<y 


— •■/ 


Fig.  240. — Median  Longitudinal  Section  of  the  Brain  of  an  Embryo  of  16.3  mm. 
br,  Anterior  brachium;  eg,  corpus  geniculatum  laterale;  cs,  corpus  striatum;  h, 
cerebral  hemisphere;  ir,  infundibular  recess;  It,  lamina  terminalis;  or,  optic  recess;  ot, 
thalamus;  p,  pineal  process;  sm,  sulcus  Monroi;  st,  hypothalamic  region;  vi,  velum 
interpositum. — (His.) 

brain  in  magnitude,  overlapping  and  completely  concealing  the 
roof  and  sides  of  the  diencephalon  and  mesencephalon  and  also  the 
anterior  surface  of  the  cerebellum.  In  this  enlargement,  however, 
the  interventricular  foramina  share  only  to  a  slight  extent,  and 
consequently  become  relatively  smaller  (Fig.  233),  forming  in  the 
adult  merely  slit-like  openings  lying  between  the  lamina  terminalis 
and  the  thalami  and  having  for  their  roof  the  anterior  portion  of  the 
velum  interpositum. 

The  velum  Interpositum — that  is  to  say,  the  roof-plate — where 


THE    TELENCEPHALON 


401 


it  forms  the  roof  of  the  interventricular  foramen,  is  prolonged  out 
upon  the  dorsal  surface  of  each  hemisphere,  and,  becoming  invag- 
inated,  forms  upon  it  a  groove.'  As  the  hemispheres,  increasing  in 
height,  develop  a  mesial  wall,  the  groove,  which  is  the  so-called 
chorioidal  fissure,  comes  to  lie  along  the  ventral  edge  of  this  wall, 
and  as  the  growth  of  the  hemispheres  continues  it  becomes  more  and 
more  elongated,  being  carried  at  first  backward  (Fig.  241),  then 
ventrally,  and  finally  forward  to  end  at  the  tip  of  the  temporal  lobe. 
After  the  establishment  of  the  grooves  the  mesenchyme  in  their 
vicinity  dips  into  them,  and,  developing  blood-vessels,  becomes  the 
chorioid  plexuses  of  the  lateral  ventricles,  and  at  first  these  plexuses 
grow  much  more  rapidly  than  the  ventricles,  and  so  fill  them  almost 
completely.  Later,  however,  the  walls 
of  the  hemispheres  gain  the  ascendancy 
in  rapidity  of  growth  and  the  plexuses 
become  relatively  much  smaller.  Since 
the  portions  of  the  roof-plate  which  form 
the  chorioidal  fissures  are  continuous 
with  the  velum  interpositum  in  the  roofs 
of  the  interventricular  foramina,  the 
chorioid  plexuses  of  the  lateral  and  third 
ventricles  become  continuous  also  at  that 
point. 


Fig.    241. 


-Median  Longi- 
tudinal Section  of  the  Brain 
of  an  Embryo  Calf  of  5  cm. 

cb,    Cerebellum;  cp,    chorioid 
plexus;  cs,  corpus  striatum;  JM, 
interventricular     foramen;     in, 
The  mode  of  growth  of  the  chorioid    hypophysis;    m,    mid-brain;   oc, 
,  optic    commissure;   t,   posterior 

fissures  seems  to  indicate  the  mode  of  part  of  the  diencephalon  — 
growth  of  the  hemispheres.  At  first  the  Wihalkovicz.) 
growth  is  more  or  less  equal  in  all  directions,  but  later  it  becomes  more 
extensive  posteriorly,  there  being  more  room  for  expansion  in  that 
direction,  and  when  further  extension  backward  becomes  difficult 
the  posterior  extremities  of  the  hemispheres  bend  ventrally  toward 
the  base  of  the  cranium,  and  reaching  this,  turn  forward  to  form  the 
temporal  lobes.  As  a  result  the  cavities  of  the  hemispheres,  the 
lateral  ventricles,  in  addition  to  being  carried  forward  to  form  an 
anterior  horn,  are  also  carried  backward  and  ventrally  to  form  the 
lateral  or  descending  horn,  and  the  corpus  striatum  likewise  extends 
26 


402  THE    TELENCEPHALON 

backward  to  the  tip  of  each  temporal  lobe  as  a  slender  process  known 
as"  the  tail  of  the  caudate  nucleus.  In  addition  to  the  anterior  and 
lateral  horns,  the  ventricles  of  the  human  brain  also  possess  posterior 
horns  extending  backward  into  the  occipital  portions  of  the  hemis- 
pheres, these  portions,  on  account  of  the  greater  persistence  of  the 
mid-brain  flexure  (see  p.  388),  being  enabled  to  develop  to  a  greater 
extent  than  in  the  lower  mammals. 

The  scheme  of  the  origin  of  parts  in  the  telencephalon  may  be 
stated  as  follows: 

Median  Part.  Hemispheres. 

„      ,    ,  f  Anterior  part  of  velum    inter-   f  _  ,    n      ..,,,. 

Roof-plate <  .  <   Moor  of  chonoidai  nssure. 

(^       positum.  [ 

r  ,        .               ...  Pallium. 

-r.         ,                                Lamina  terminahs.  _ 

Dorsal  zones ■(_...  <    Corpus  striatum. 

Optic  evaginations.  _,,             ,  ,       .                  .. 

>             .                               ,           .  Olfactory  lobes  (see  p.  406) 

Anterior  part  of  hypothalamic  [ 

Ventral  zones <        region. 

[  Anterior  part  of  tuber  cinereum. 

The  Convolutions  of  the  Hemispheres. — The  growth  of  the 
hemispheres  to  form  the  voluminous  structures  found  in  the  adult 
depends  mainly  upon  an  increase  of  size  of  the  pallium.  The 
corpus  striatum,  although  it  takes  part  in  the  elongation  of  each 
hemisphere,  nevertheless  does  not  increase  in  other  directions  as 
rapidly  and  extensively  as  the  pallium,  and  hence,  even  in  very  early 
stages,  a  depression  appears  upon  the  surface  of  the  hemispheres 
where  the  corpus  is  situated  (Fig.  242).  This  depression  is  the 
lateral  cerebral  fossa,  and  for  a  considerable  period  it  is  the  only  sign 
of  inequality  of  growth  on  the  outer  surfaces  of  the  hemispheres. 
Upon  the  mesial  surfaces,  however,  at  about  the  time  that  the 
choroid  fissure  appears,  another  linear  depression  is  formed  dorsal 
to  the  chorioid,  and  when  fully  formed  extends  from  in  front  of  the 
interventricular  foramen  to  the  tip  of  the  temporal  lobe  (Fig.  244,  h). 
It  affects  the  entire  thickness  of  the  pallial  wall  and  consequently 
produces  an  elevation  upon  the  inner  surface,  a  projection  into  the 
cavity  of  the  ventricle  which  is  known  as  the  hippocampus,  whence 


THE  CEREBRAL  CONVOLUTIONS 


403 


the  fissure  may  be  termed  the  hippocampal  fissure.  The  portion  of 
the  pallium  which  intervenes  between  this  fissure  and  the  chorioidal 
forms  what  is  known  as  the  dentate  gyrus. 

Toward  the  end  of  the  third  or  the  beginning  of  the  fourth  month 
two  prolongations  arise  from  the  fissure  just  where  it  turns  to  be 
continued  into  the  temporal  lobe,  and  these,  extending  posteriorly, 
give  rise  to  the  parieto-occipital  and  calcarine  fissures.  Like  the 
hippocampal,  these  fissures  produce  elevations  upon  the  inner 
surface  of  the  pallium,  that  formed  by  the  parieto-occipital  early 
disappearing,  while  that  pro- 
duced by  the  calcarine  persists 
to  form  the  calcar  {hippocam- 
pus minor)  of  adult  anatomy. 

The  three  fissures  just 
described,  together  with  the 
chorioidal  and  the  lateral 
cerebral  fossa,  are  all  formed 
by  the  beginning  of  the  fourth 
month  and  all  the  fissures 
affect  the  entire  thickness  of 
the  wall  of  the  hemisphere, 
and  hence  have  been  termed 
the  primary  or  total  fissures. 
Until  the  beginning  of  the  fifth 

month  they  are  the  only  fissures  present,  but  at  that  time  secondary 
fissures,  which,  with  one  exception,  are  merely  furrows  of  the  sur- 
face of  the  pallium,  make  their  appearance  and  continue  to  form 
until  birth  and  possibly  later.  Before  considering  these,  however, 
certain  changes  which  occur  in  the  neighborhood  of  the  lateral 
cerebral  fossa    may    be  described. 

The  fossa  is  at  first  a  triangular  depression  situated  above  the 
temporal  lobe  on  the  surface  of  the  hemisphere.  During  the  fourth 
month  it  deepens  considerably,  so  that  its  upper  and  lower  margins 
become  more  pronounced  and  form  projecting  folds,  and,  during 
the  fifth  month,  these  two  folds  approach  one  another  and  eventually 


Fig.  242. — Brain  of  an  Embryo  of  the 

Fourth  Month. 
c,   Cerebellum;  p,  pons;  s,  lateral   cerebral 
fossa. 


404 


TEE    CEREBRAL    CONVOLUTIONS 


cover  in  the  floor  of  the  fossa  completely,  the  groove  which  marks 
the  line  of  their  contact  forming  the  lateral  cerebral  fissure,  while  the 
floor  of  the  fossa  becomes  known  as  the  insula. 

The  first  of  the  secondary  fissures  to  appear  is  the  sulcus  cinguli, 
which  is  formed  about  the  middle  of  the  fifth  month  on  the  mesial 
surface  of  the  hemispheres,  lying  parallel  to  the  anterior  portion  of 
the  hippocampal  fissure  and  dividing  the  mesial  surface  into  the 
gyri  marginalis  and  fornicatus.  A  little  later,  at  the  beginning  of 
the  sixth  month,  several  other  fissures  make  their  appearance  upon 


ptc 


Fig.  243. — Cerebral  Hemisphere  oe  an  Embryo  of  about  the  Seventh  Month. 
fs,  Superior  frontal  sulcus;  ip,  interparietal;  IR,  insula;  pet,  inferior  pre-central;  pes, 
superior    pre-central;  ptc,  post-central;  R,  central;  S,  lateral;    t1,    first    temporal. — 
{Cunningham  ) 


the  outer  surface  of  the  pallium,  the  chief  of  these  being  the  central 
sulcus,  the  inter-parietal,  the  pre-  and  post-central,  and  the  temporal 
sulci,  the  most  ventral  of  these  last  running  parallel  with  the  lower 
portion  of  the  hippocampal  fissure  and  differing  from  the  others  in 
forming  a  ridge  on  the  wall  of  the  ventricle  termed  the  collateral 
eminence,  whence  the  fissure  is  known  as  the  collateral.  The  position 
of  most  of  these  fissures  may  be  seen  from  Fig.  243,  and  for  a  more 


THE    CORPUS    CALLOSUM 


405 


complete  description  of  them  reference  may  be  had  to  text-books  of 
descriptive  anatomy. 

In  later  stages  numerous  tertiary  fissures  make  their  appearance 
and  mask  more  or  less  extensively  the  secondaries,  than  which  they 
are,  as  a  rule,  much  more  inconstant  in  position  and  shallower. 
The  Corpus  Callosum  and  Fornix. — While  these  fissures  have  been 
forming,  important  structures  have  developed  in  connection  with 
the  lamina  terminalis.  Up  to  about  the  fourth  month  the  lamina 
is  thin  and  of  nearly  uniform  thickness  throughout,  but  at  this  time 
it  begins  to  thicken  near  its  dorsal  edge  and  fibers  appear  in  the 
thickening.  These  fibers  belong  to  three  sets.  In  the  first  place, 
certain  of  them  arise  in  connection  with  the  olfactory  tracts  (see  p. 
407)  and  from  the  region  of  the  hippocampal  gyrus,  which  is  also 
associated  with  the  olfactory  sense,  and,  passing  through  tbe  sub- 
stance of  the  lamina  terminalis,  they  extend  across  the  median  line 
to  the  corresponding  regions  of  the  opposite  cerebral  hemisphere. 
They  are  therefore  commissural  fibers  and  form  what  is  termed  the 
anterior  commissure  (Figs.  244,  ca  and  245,  ac).  Secondly,  fibers, 
which  have  their  origin  from  the  cells  of  the  hippocampus,  develop 
along  the  chorioidal  edge  of  that  structure,  forming  what  is  termed 
the  fimbria.  They  follow  along  the  edge  of  the  chorioidal  fissure 
and,  when  this  reaches  the  interventricular  foramen,  they  enter  as 
the  pillars  of  the  fornix  (Figs.  244,  cf;  Fig.  245,/)  the  substance  of  the 
lamina  terminalis  and,  passing  ventrally  in  it,  eventually  reach  the 
hypothalamic  region,  where  they  terminate  in  the  corpora 
mammillaria. 

Thirdly,  as  the  mantle  develops  fibers  radiate  from  all  parts  of 
it  toward  the  dorsal  portion  of  the  lamina  terminalis  and  traversing 
it  are  distributed  to  the  corresponding  portions  of  the  mantle  of  the 
opposite  side.  There  fibers  are  also  commissural  in  character  and 
form  the  corpus  callosum  (Figs.  244  and  245,  cc).  With  the  develop- 
ment of  these  three  sets  of  fibers  and  especially  those  forming  the 
corpus  callosum,  the  dorsal  portion  of  the  lamina  terminalis  be- 
comes enlarged  so  as  to  form  a  triangular  area  extending  between 
the  two  cerebral  hemispheres  (Fig.  245),  the  corpus  callosum  form- 


4<o6 


THE    CORPUS    CALLOSUM 


ing  its  dorsal  portion  and  base,  which  is  directed  anteriorly,  the 
pillars  of  the  fornix  its  ventral  portion,  while  the  anterior  commissure 
occupies   its   ventral   anterior   angle. 

The  portion  of  the  triangle  included  between  the  callosum  and 
the  fornix  remains  thin  and  forms  the  septum  pellucidum,  and  a  split 
occurring  in  the  center  of  this  gives  rise  to  the  so-called^///*  ventricle, 

which,  from  its  mode  of  forma- 
tion, is  a  completely  closed  cav- 
ity and  is  not  lined  with  epen- 
dymal  tissue  of  the  same  nature 
as  that  found  in  the  other  ven- 
tricles. 

Owing  to  the  very  consider- 
able size  reached  by  the  trian- 
gular area  whose  history  has  just 
been  described,  important 
changes  are  wrought  in  the  ad- 
joining portions  of  the  mesial 
surface  of  the  hemispheres.  Be- 
fore the  development  of  the  area 
the  gyrus  dentatus  and  the  hip- 
pocampus extend  forward  into 
the  anterior  portion  of  the  hem- 
ispheres (Fig.  244),  but  on  ac- 
count of  their  position  they  be- 
come encroached  upon  by  the 
enlargement  of  the  corpus  callo- 
sum, with  the  result  that  the  hippocampus  becomes  practically 
obliterated  in  that  portion  of  its  course  which  lies  in  the  region 
occupied  by  the  corpus  callosum,  its  fissure  in  this  region  becoming 
known  as  the  callosal  fissure,  while  the  corresponding  portions 
of  the  dentate  gyrus  become  reduced  to  narrow  and  insignificant 
bands  of  nerve  tissue  which  rest  upon  the  upper  surface  of  the  corpus 
callosum  and  are  known  as  the  lateral  longitudinal  stria. 

The  Olfactory  Lobes. — At  the  time  when  the  cerebral  hemispheres 


Fig.  244. — Median  Longitudinal  Sec- 
tion or  the  Brain  of  an  Embryo  of 
Four  Months. 

c,  Calcarine  fissure;  ca,  anterior  com- 
missure; cc,  corpus  callosum;  cf.  chorioidal 
fissure;  dg,  dentate  gyrus;  fm,  interven- 
tricular foramen;  h,  hippocampal  fissure; 
po,  parieto-o  c  c  i  p  i  t  a  1  fissure. — (Mihal- 
kovicz.) 


THE    OLFACTORY   LOBES 


407 


begin  to  enlarge — that  it  to  say,  at  about  the  fourth  week — a  slight 
furrow,  which  appears  on  the  ventral  surface  of  each  anteriorly, 
marks  off  an  area  which,  continuing  to  enlarge  with  the  hemispheres, 
gradually  becomes  constricted  off  from  them  to  form  a  distinct  lobe- 
like structure,  the  olfactory  lobe  (Fig.  233,  VI  3).  In  most  of  the 
lower  mammalia  these  lobes 
reach  a  very  considerable  size, 
and  consequently  have  been 
regarded  as  constituting  an 
additional  division  of  the 
brain,  known  as  the  rhinen- 
cephalon,  but  in  man  they 
remain  smaller,  and  although 
they  are  at  first  hollow,  con- 
taining prolongations  from  the 
lateral  ventricles,  the  cavities 
later  on  disappear  and  the 
lobes  become  solid.  Each 
lobe  becomes  differentiated 
into  two  portions,  its  terminal 
portion  becoming  converted 
into  the  club-shaped  struc- 
ture, the  olfactory  bulb  and  stalk,  while  its  proximal  portion  gives 
rise  to  the  olfactory  tracts,  the  trigone,  and  the  anterior  perforated 
substance. 

Histogenesis  of  the  Cerebral  Cortex. — A  satisfactory  study  of  the 
histogenesis  of  the  cortex  has  not  yet  been  made.  In  embryos  of 
three  months  a  marginal  velum  is  present  and  probably  gives  rise 
to  the  stratum  zonale  of  the  adult  brain;  beneath  this  is  a  cellular 
layer,  perhaps  representing  the  mantle  layer;  beneath  this,  again,  a 
layer  of  nerve-fibers  is  beginning  to  appear,  representing  the  white 
substance  of  the  pallium;  and,  finally,  lining  the  ventricle  is  an 
ependymal  layer.  In  embryos  of  the  fifth  month,  toward  the  in- 
nermost part  of  the  second  layer,  cells  are  beginning  to  differentiate 
into  the  large  pyramidal  cells,  but  almost  nothing  is  known  as  to  the 


Fig.  245. — Median  Longitudinal  Section 
of  the  Brain  oe  an  Embryo  of  the  Fifth 
Month. 

ac,  Anterior  commissure;  cc,  corpus  callo- 
sum;  dg,  dentate  gyrus;/,  fornix;  i,  infundib- 
ulum;  mc,  intermediate  mass;  si,  septum 
pellucidum;  vi,  velum  interpositum. — (Mihal- 

kovicz.) 


408  THE    SPINAL  NERVES 

origin  of  the  other  layers  recognizable  in  the  adult  cortex,  nor  is  it 
known  whether  any  migration,  similar  to  what  occurs  in  the  cere- 
bellar cortex,  takes  place.  The  fibers  of  the  white  substance  do  not 
begin  to  acquire  their  myelin  sheaths  until  toward  the  end  of  the 
ninth  month,  and  the  process  is  not  completed  until  some  time  after 
birth  (Flechsig),  while  the  fibers  of  the  cortex  continue  to  undergo 
myelination  until  comparatively  late  in  life  (Kaes). 

The  Development  of  the  Spinal  Nerves. — It  has  already  been 
seen  that  there  is  a  fundamental  difference  in  the  mode  of  develop- 
ment of  the  two  roots  of  which  the  typical  spinal  nerves  are  composed, 
the  ventral  root  being  formed  by  axis-cylinders  which  arise  from 
neuroblasts  situated  within  the  substance  of  the  spinal  cord,  while 
the  dorsal  roots  arise  from  the  cells  of  the  neural  crests,  their  axis- 
cylinders  growing  into  the  substance  of  the  cord  while  their  dendrites 
become  prolonged  peripherally  to  form  the  sensory  fibers  of  the 
nerves.  Throughout  the  thoracic,  lumbar  and  sacral  regions  of  the 
cord  the  fibers  which  grow  out  from  the  anterior  horn  cells  converge 
to  form  a  single  nerve-root  in  each  segment,  but  in  the  cervical  region 
fibers  which  arise  from  the  more  laterally  situated  neuroblasts  make 
their  exit  from  the  cord  independently  of  the  more  ventral  neuro- 
blasts and  form  the  roots  of  the  spinal  accessory  nerve  (see  p.  416). 
In  the  cervical  region  there  are  accordingly  three  sets  of  nerve-roots, 
the  dorsal,  lateral,  and  ventral  sets. 

In  a  typical  spinal  nerve,  such  as  one  of  the  thoracic  series,  the 
dorsal  roots  as  they  grow  peripherally  pass  ventrally  as  well  as  out- 
ward, so  that  they  quickly  come  into  contact  with  the  ventral  roots 
with  whose  fibers  they  mingle,  and  the  mixed  nerve  so  formed  soon 
after  divides  into  two  trunks,  a  dorsal  one,  which  is  distributed  to  the 
dorsal  musculature  and  integument,  and  a  larger  ventral  one.  The 
ventral  division  as  it  continues  its  outward  growth  soon  reaches  the 
dorsal  angle  of  the  pleuro-peritoneal  cavity,  where  it  divides,  one 
branch  passing  into  the  tissue  of  the  body- wall  while  the  other  passes 
into  the  splanchnic  mesoderm.  The  former  branch,  continuing  its 
onward  course  in  the  body- wall,  again  divides,  one  branch  becoming 
the  lateral  cutaneous  nerve,  while  the  other  continues  inward  to 


THE    CRANIAL   NERVES  409 

terminate  in  the  median  ventral  portion  of  the  body  as  the  anterior 
cutaneous  nerve.  The  splanchnic  branch  forms  a  ramus  communi- 
cans  to  the  sympathetic  system  and  will  be  considered  more  fully 
later  on. 

The  conditions  just  described  are  those  which  obtain  throughout 
the  greater  part  of  the  thoracic  region.  Elsewhere  the  fibers  of  the 
ventral  divisions  of  the  nerves  as  they  grow  outward  tend  to  separate 
from  one  another  and  to  become  associated  with  the  fibers  of  adja- 
cent nerves,  giving  rise  to  plexuses.  In  the  regions  where  the  limbs 
occur  the  formation  of  the  plexuses  is  also  associated  with  a  shifting 
of  the  parts  to  which  the  nerves  are  supplied,  a  factor  in  plexus  forma- 
tion which  is,  however,  much  more  evident  from  comparative 
anatomical  than  from  embryological  studies. 

The  Development  of  the  Cranial  Nerves.— During  the  last 
thirty  years  the  cranial  nerves  have  received  a  great  deal  of  attention 
in  connection  with  the  idea  that  an  accurate  knowledge  of  their 
development  would  afford  a  clue  to  a  most  vexed  problem  of  verte- 
brate morphology,  the  metamerism  of  the  head.  That  the  meta- 
merism which  was  so  pronounced  in  the  trunk  should  extend  into  the 
head  was  a  natural  supposition,  strengthened  by  the  discovery  of 
head-cavities  in  the  lower  vertebrates  and  by  the  indications  of 
metamerism  seen  in  the  branchial  arches,  and  the  problem  which 
presented  itself  was  the  correlation  of  the  various  structures  belonging 
to  each  metamere  and  the  determination  of  the  modifications  which 
they  had  undergone  during  the  evolution  of  the  head. 

In  the  trunk  region  a  nerve  forms  a  conspicuous  element  of  each 
metamere  and  is  composed,  according  to  what  is  known  as  Bell's 
law,  of  a  ventral  or  efferent  and  a  dorsal  or  afferent  root.  Until 
comparatively  recently  the  study  of  the  cranial  nerves  has  been 
dominated  by  the  idea  that  it  was  possible  to  extend  the  application 
of  Bell's  law  to  them  and  to  recognize  in  the  cranial  region  a  number 
of  nerve  pairs  serially  homologous  with  the  spinal  nerves,  some  of 
them,  however,  having  lost  their  afferent  roots,  while  in  others  a  dis- 
location, as  it  were,  of  the  two  roots  had  occurred. 

The  results  obtained  from  investigation  along  this  line  have  not, 


4IO  .         THE    CRANIAL  NERVES 

however,  proved  entirely  satisfactory,  and  facts  have  been  elucidated 
which  seem  to  show  that  it  is  not  possible  to  extend  Bell's  law,  in  its 
usual  form  at  least,  to  the  cranial  nerves.  It  has  been  found  that 
it  is  not  sufficient  to  recognize  simply  afferent  and  efferent  roots, 
but  these  must  be  analyzed  into  further  components,  and  when  this 
is  done  it  is  found  that  in  the  series  of  cranial  nerves  certain  com- 
ponents occur  which  are  not  represented  in  the  nerves  of  the  spinal 
series. 

Before  proceeding  to  a  description  of  these  components  it  will  be 
well  to  call  attention  to  a  matter  already  alluded  to  in  a  previous 
chapter  (p,  84)  in  connection  with  the  segmentation  of  the  meso- 
derm of  the  head.  It  has  been  pointed  out  that  while  there  exist 
"head-cavities"  which  are  serially  homologous  with  the  mesodermal 
somites  of  the  trunk,  there  has  been  imposed  upon  this  primary 
cranial  metamerism  a  secondary  metamerism  represented  by 
the  branchiomeres  associated  with  the  branchial  arches,  and, 
it  may  be  added,  this  secondary  metamerism  has  become  the  more 
prominent  of  the  two,  the  primary  one,  as  it  developed,  gradually 
slipping  into  the  background  until,  in  the  higher  vertebrates,  it  has 
become  to  a  very  considerable  extent  rudimentary.  In  accordance 
with  this  double  metamerism  it  is  necessary  to  recognize  two  sets  of 
cranial  muscles,  one  derived  from  the  cranial  myotomes  and  repre- 
sented by  the  muscles  of  the  eyeball,  and  one  derived  from  the 
branchiomeric  mesoderm,  and  it  is  necessary  also  to  recognize 
for  these  two  sets  of  muscles  two  sets  of  motor  nerves,  so 
that,  with  the  dorsal  or  sensory  nerve-roots,  there  are  altogether 
three  sets  of  nerve-roots  in  the  cranial  region  instead  of  only  two,  as 
in  the  spinal  region. 

These  three  sets  of  roots  are  readily  recognizable  both  in  the  em- 
bryonic and  in  the  adult  brain,  especially  if  attention  be  directed  to 
the  cell  groups  or  nuclei  with  which  they  are  associated  (Fig.  246). 
Thus  there  can  be  recognized:  (1)  a  series  of  nuclei  from  which 
nerve-fibers  arise,  situated  in  the  floor  of  the  fourth  ventricle  and 
aquaeduct  close  to  the  median  line  and  termed  the  ventral  motor 
nuclei;  (2)  a  second  series  of  nuclei  of  origin,  situated  more  laterally 


THE    CRANIAL   NERVES 


411 


and  in  the  substance  of  the  formatio  reticularis,  and  known  as  the 
lateral  motor  nuclei;  and  (3)  a  series  of  nuclei  in  which  afferent  nerve- 
fibers  terminate,  situated  still  more  laterally  in  the  floor  of  the  ven- 
tricle and  forming  the  dorsal  or  sensory  nuclei.  None  of  the  twelve 
cranial  nerves  usually  recognized  in  the  text-books  contains  fibers 
associated  with  all  three  of  these  nuclei;  the  fibers  from  the  lateral 
motor  nuclei  almost  invariably  unite  with  sensory  fibers  to  form  a 


Fig.  246. — Transverse  Section  through  the  Medulla  Oblongata  of  an 
Embryo  of  10  mm.,  showing  the  Nuclei  of  Origin  of  the  Vagus  (X)  and  Hypo- 
glossal (XII)  Nerves. — (His.) 

mixed  nerve,  but  those  from  all  the  ventral  motor  nuclei  form  inde- 
pendent roots,  while  the  olfactory  and  auditory  nerves  alone,  of  all 
the  sensory  roots  (omitting  for  the  present  the  optic  nerve),  do  not 
contain  fibers  from  either  of  the  series  of  motor  nuclei.  The  relations 
of  the  various  cranial  nerves  to  the  nuclei  may  be  seen  from  the 
following  table,  in  which  the  +  sign  indicates  the  presence  and  the 
—  sign  the  absence  of  fibers  from  the  nuclear  series  under  which  it 
stands': 


412 


THE    CRANIAL   NERVES 


Number 

Name 

Ventral  Motor 

Lateral  Motor 

Sensory 

I. 

Olfactory. 

_ 



+ 

III. 

Oculomotor. 

+ 

- 

- 

TV. 

Trochlear. 

+ 

- 

- 

V. 

Trigeminus. 

- 

+ 

+ 

VI 

Abducens. 

+ 

- 

- 

VII. 

Facial. 

- 

+ 

+ 

VIII. 

Auditory. 

- 

- 

+ 

IX. 

Glossopharyngeal. 

- 

+ 

+ 

X. 

XI. 

Vagus.                      1 
Spinal  Accessory.  J 

+ 

+ 

Two  nerves — namely,  the  second  and  twelfth — have  been  omitted 
from  the  above  table.  Of  these,  the  second  or  optic  nerve  undoubt- 
edly belongs  to  ah  entirely  different  category  from  the  other  periph- 
eral nerves,  and  will  be  considered  in  the  following  chapter  in 
connection  with  the  sense-organ  with  which  it  is  associated  (see 
especially  p.  460).  The  twelfth  or  hypoglossal  nerve,  on  the  other 
hand,  really  belongs  to  the  spinal  series  and  has  only  secondarily 
been  taken  up  into  the  cranial  region  in  the  higher  vertebrates.  It 
has  already  been  seen  (p.  170)  that  the  bodies  of  four  vertebrae  are 
included  in  the  basioccipital  bone,  and  that  three  of  the  nerves 
corresponding  to  these  vertebrae  are  represented  in  the  adult  by  the 
hypoglossal  and  the  fourth  by  the  first  cervical  or  suboccipital  nerve. 
The  dorsal  roots  of  the  hypoglossal  nerves  seem  to  have  almost 
disappeared,  although  a  ganglion  has  been  observed  in  embryos  of 
7  and  10  mm.  in  the  posterior  part  of  the  hypoglossal  region  (His), 
and  probably  represents  the  dorsal  root  of  the  most  posterior  portion 
of  the  hypoglossal  nerve.  This  ganglion  disappears,  as  a  rule,  in 
later  stages,  and  it  is  interesting  to  note  that  the  ganglion  of  the 
suboccipital  nerve  is  also  occasionally  wanting  in  the  adult  condition. 
The  hypoglossal  roots  are  to  be  regarded,  then,  as  equivalent  to  the 
ventral  roots  of  the  cervical  spinal  nerves,  and  the  nuclei  from 
which  they  arise  lie  in  series  with  the  cranial  ventral  motor  roots,  a 


THE    CRANIAL   NERVES  413 

fact  which  indicates  the  equivalency  of  these  latter  with  the  fibers 
which  arise  from  the  neuroblasts  of  the  anterior  horns  of  the  spinal 
cord. 

The  equivalents  of  the  lateral  motor  roots  may  more  conveniently 
be  considered  later  on,  but  it  may  be  pointed  out  here  that  these  are 
the  fibers  which  are  distributed  to  the  muscles  of  the  branchiomeres. 
In  the  case  of  the  sensory  nerves  a  further  analysis  is  necessary 
before  their  equivalents  in  the  spinal  series  can  be  determined. 
For  this  the  studies  which  have  been  made  in  recent  years  of  the 
components  entering  into  the  cranial  nerves  of  the  amphibia  (Strong) 
and  fishes  (Herrick)  must  supply  a  basis,  since  as  yet  a  direct  analysis 
of  the  mammalian  nerves  has  not  been  made.  In  the  forms  named 
it  has  been  found  that  three  different  components  enter  into  the 
formation  of  the  dorsal  roots  of  the  cranial  nerves:  (i)  fibers  belong- 
ing to  a  general  cutaneous  or  somatic  sensory  system,  distributed  to 
the  skin  without  being  connected  with  any  special  sense-organs;  (2) 
fibers  belonging  to  what  is  termed  the  communis  or  viscerosensory 
system,  distributed  to  the  walls  of  the  mouth  and  pharyngeal  region 
and  to  special  organs  found  in  the  skin  of  the  same  character  as 
those  occurring  in  the  mouth;  and  (3)  fibers  belonging  to  a  special 
set  of  cutaneous  sense-organs  largely  developed  in  the  fishes  and 
known  as  the  organs  of  the  lateral  line. 

The  fibers  of  the  somatic  sensory  system  converge  to  a  group  of 
cells,  situated  in  the  lateral  part  of  the  floor  of  the  fourth  ventricle 
and  forming  what  is  termed  the  trigeminal  lobe,  and  also  extend 
posteriorly  in  the  substance  of  the  medulla  (Fig.  247),  forming  what 
has  been  termed  the  spinal  root  of  the  trigeminus  and  terminating 
in  a  column  of  cells  which  represents  the  forward  continuation  of  the 
posterior  horn  of  the  cord.  In  the  fishes  and  amphibia  fibers 
belonging  to  this  system  are  to  be  found  in  the  fifth,  seventh,  and 
tenth  nerves,  but  in  the  mammalia  their  distribution  has  apparently 
become  more  limited,  being  confined  almost  exclusively  to  the 
trigeminus,  of  whose  sensory  divisions  they  form  a  very  considerable 
part.  Since  the  cells  around  which  the  fibers  of  the  spinal  root  of  the 
trigeminus  terminate  are  the  forward  continuations  of  the  posterior 


414 


THE    CRANIAL  NERVES 


horn  of  the  cord,  it  seems  probable  that  the  fibers  of  this  system 
are  the  cranial  representatives  of  the  posterior  roots  of  the  spinal 
nerves,  which,  it  may  be  noted,  are  also  somatic  in  their  distribution. 
The  fibers  of  the  viscero-sensory  system  are  found  in  the  lower 
forms  principally  in  the  ninth  and  tenth  nerves  (see  Fig.  247), 
although  groups  of  them  are  also  incorporated  in  the  seventh  and 
fifth.  They  converge  to  a  mass  of  cells,  known  as  the  lobus  vagi, 
and  like  the  first  set  are  also  continued  down  the  medulla  to  form 


rix 


Fig.  247.— Diagrams  showing  the  Sensory  Components  of  the  Cranial  Nerves 

of  a  Fish  (Menidia) . 
The  somatic  sensory  system  is  unshaded,  the  viscero-sensory  is  cross-hatched,  and 
the  lateral  line  system  is  black,  asc.v,  Spinal  root  of  trigeminus;  brx,  branchial  branches 
of  vagus;  Ix,  lobus  vagi;  ol,  olfactory  bulb;  op,  optic  nerve;  rc.x,  cutaneous  branch  of  the 
vagus;  rix,  intestinal  branch  of  vagus;  rl,  lateral  line  nerve;  rl.acc,  accessory  lateral 
line  nerve;  ros,  superficial  ophthalmic;  rp,  ramus  palatinus  of  the  facial;  thy,  hyomandib- 
ular  branch  of  the  facial;  t.inf,  infraorbital  nerve. — {Herrick.) 

a  tract  known  as  the  fasciculus  solitarius  or:  fasciculus  communis.  In 
the  mammalia  the  system  is  represented  by  the  sensory  fibers  of  the 
glosso-pharyngeo-vagus  set  of  nerves,  of  which  it  represents  prac- 
tically the  entire  mass;  by  the  sensory  fibers  of  the  facial  arising  from 
the  geniculate  ganglion  and  included  in  the  chorda  tympani  and 
probably  also  the  great  superficial  petrosal;  and  also,  probably,  by 


THE    CRANIAL   NERVES  415 

the  lingual  branch  of  the  trigeminus.  Furthermore,  since  the 
mucous  membrane  of  the  palate  is  supplied  by  branches  from  the 
trigeminus  which  pass  by  way  of  the  spheno-palatine  (Meckel's) 
ganglion,  and  the  same  region  is  supplied  in  lower  forms  by  a  pala- 
tine branch  from  the  facial,  it  seems  probable  that  the  palatine  nerves 
of  the  mammalia  are  also  to  be  assigned  to  this  system.*  If  this 
be  the  case,  a  very  evident  clue  is  afforded  to  the  homologies  of  the 
system  in  the  spinal  nerves,  for  since  the  spheno-palatine  ganglion 
is  to  be  regarded  as  part  of  the  sympathetic  system,  the  sensory 
fibers  which  pass  from  the  viscera  to  the  spinal  cord  by  way  of  the 
sympathetic  system  (p.  420)  present  relations  practically  identical 
with  those  of  the  palatine  nerves. 

Finally,  with  regard  to  the  system  of  the  lateral  line,  there  seems 
but  little  doubt  that  it  has  no  representation  whatsoever  in  the  spinal 
nerves.  It  is  associated  with  a  peculiar  system  of  cutaneous  sense- 
organs  found  only  in  aquatic  or  marine  animals,  and  also  with  the 
auditory  and  possibly  the  olfactory  organs,  the  former  of  which  are 
certainly  and  the  latter  possibly  primarily  parts  of  the  lateral  line 
system  of  organs.  The  organs  are  principally  confined  to  the  head, 
although  they  also  extend  upon  the  trunk,  where  they  are  followed 
by  a  branch  from  the  vagus  nerve,  the  entire  system  being  accordingly 
supplied  by  cranial  nerves.  In  the  fishes,  in  which  the  development 
of  the  organs  is  at  a  maximum,  fibers  belonging  to  the  system  are 
found  in  all  the  branchiomeric  nerves  and  all  converge  to  a  portion 
of  the  medulla  known  as  the  tuberculum  acusticum.  In  the  Mam- 
malia, with  the  disappearance  of  the  lateral  line  organs  there  has 
been  a  disappearance  of  the  associated  nerves,  and  the  only  certain 
representative  of  the  system  which  persists  is  the  auditory  nerve. 

The  table  given  on  page  412  may  now  be  expanded  as  follows, 
though  it  must  be  recognized  that  such  an  analysis  of  the  mammalian 
nerves  is  merely  a  deduction  from  what  has  been  observed  in  lower 

*  The  fact  that  the  palatine  branches  are  associated  with  the  trigeminus  in  the 
Mammalia  and  with  the  facial  in  the  Amphibia  is  readily  explained  by  the  fact  that 
in  the  latter  the  Gasserian  and  geniculate  ganglia  are  not  always  separated,  so  that 
it  is  possible  for  fibers  originating  from  the  compound  ganglion  to  pass  into  either 


416 


THE    CRANIAL   NERVES 


forms,  and  may  require  some  modifications  when  the  components 
have  been  subjected  to  actual  observation: 


Nerve 

Ventral 

Lateral 

Somatic 

Visceral 

Lateral 

Motor 

Motor 

Sensory 

Sensory 

Line 

I. 

_ 





+ 

III. 

+ 

- 

- 

- 

- 

IV. 

+ 

- 

- 

- 

- 

V. 

- 

+ 

+ 

+ 

- 

VI. 

+ 

- 

- 

- 

- 

vii. 

- 

+ 

- 

+ 

- 

VIII. 

- 

- 

- 

- 

+ 

IX.] 

X. 

- 

+ 

+ 

+ 

- 

XL  J 

XII. 

+ 

- 

- 

- 

- 

Spinal. 

+ 

(?) 

+ 

+ 

An  additional  word  is  necessary  concerning  the  spinal  accessory 
nerve,  for  it  presents  certain  interesting  relations  which  possibly 
furnish  a  clue  to  the  spinal  equivalents  of  the  lateral  motor  roots. 
In  the  first  place,  the  neuroblasts  which  give  rise  to  those  fibers  of 
the  nerve  which  come  from  the  spinal  cord  are  situated  in  the  dorsal 
part  of  the  ventral  zones.  As  the  nuclei  of  origin  are  traced  anter- 
iorly they  will  be  found  to  change  their  position  somewhat  as  the 
medulla  is  reached  and  eventually  come  to  lie  in  the  reticular  forma- 
tion, the  most  anterior  of  them  being  practically  continuous  with 
the  motor  nucleus  of  the  vagus.  Indeed,  it  seems  that  the  spinal 
accessory  nerve  is  properly  to  be  regarded  as  an  extension  of  the 
vagus  downward  into  the  cervical  region  (Furbringer,  Streeter), 
a  process  which  reaches  its  greatest  development  in  the  mammalia 
and  seems  to-stand  in  relation  to  the  development  of  those  portions 
of  the  trapezius  and  sterno-mastoid  muscles  which  are  supplied  by 
the  spinal  accessory  nerve. 

It  is  believed  that  the  white  rami  communicantes  which  pass 
from  the  spinal  cord  to  the  thoracic  and  upper  lumbar  sympathetic 


THE    CRANIAL   NERVES  417 

ganglia  arise  from  cells  situated  in  the  dorso-lateral  portions  of  the 
ventral  horns,  and  it  is  noteworthy  that  white  rami  are  wanting  in 
the  region  in  which  the  spinal  accessory  nerve  occurs.  Since  this 
nerve  represents  a  cranial  lateral  motor  root  the  temptation  is  great 
to  regard  the  cranial  lateral  motor  roots  as  equivalent  to  the  white 
rami  of  the  cord,  and  this  temptation  is  intensified  when  it  is  recalled 
that  there  are  both  embryological  and  topographical  reasons  for 
regarding  the  branchiomeric  muscles,  to  which  the  cranial  lateral 
motor  nerves  are  supplied,  as  equivalent  to  the  visceral  muscles  of 
the  trunk.  But  in  view  of  the  fact  that  a  sympathetic  neurone  is 
always  interposed  between  a  white  ramus  fiber  and  the  visceral 
musculature  (see  Fig.  249),  while  the  lateral  motor  fibers  connect 
directly  with  the  branchiomeric  musculature,  it  seems  advisable  to 
await  further  studies  before  yielding  to  the  temptation. 

As  regards  the  actual  development  of  the  cranial  nerves,  they 
follow  the  general  law  which  obtains  for  the  spinal  nerves,  the 
motor  fibers  being  outgrowths  from  neuroblasts  situated  in  the 
walls  of  the  neural  tube,  while  the  sensory  nerves  are  outgrowths 
from  the  cells  of  ganglia  situated  without  the  tube.  In  the  lower 
vertebrates  a  series  of  thickenings,  known  as  the  suprabranchial  pla- 
codes, are  developed  from  the  ectoderm  along  a  line  corresponding 
with  the  level  of  the  auditory  invagination,  while  on  a  line  corre- 
sponding with  the  upper  extremities  of  the  branchial  clefts  another 
series  occurs  which  has  been  termed  that  of  the  epibranchial  placodes, 
and  with  both  of  these  sets  of  placodes  the  cranial  nerves  are  in 
connection.  In  the  human  embryo  epibranchial  placodes  have 
been  found  in  connection  with  the  fifth,  seventh,  ninth  and  tenth 
nerves,  to  whose  ganglia  they  contribute  cells.  The  suprabranchial 
placodes,  which  in  the  lower  vertebrates  are  associated  with  the 
lateral  line  nerves,  are  unrepresented  in  man,  unless,  as  has  been 
maintained,  the  sense-organs  of  the  internal  ear  are  their 
representatives. 

From  what  has  been  said  above  it  is  clear  that  the  usual  arrangement 
of  the  cranial  nerves  in  twelve  pairs  does  not  represent  their  true  relation- 
ships with  one  another.     The  various  pairs  are  serially  homologous  neither 

27 


418  THE    SYMPATHETIC    SYSTEM 

with  one  another  nor  with  the  typical  spinal  nerves,  nor  can  they  be 
regarded  as  representing  twelve  cranial  segments.  Indeed,  it  would  seem 
that  comparatively  little  information  with  regard  to  the  number  of 
myotomic  segments  which  have  fused  together  to  form  the  head  is  to  be 
derived  from  the  cranial  nerves,  for  while  there  are  only  four  of  these 
nerves  which  are  associated  with  structures  equivalent  to  the  mesodermic 
somites  of  the  trunk,  a  much  greater  number  of  head  cavities  or  meso- 
dermic somites  has  been  observed  in  the  cranial  region  of  the  embryos 
of  the  lower  vertebrates,  Dohrn,  for  instance,  having  found  nineteen  and 
Killian  eighteen  in  the  cranial  region  of  Torpedo.  Furthermore,  it  is  not 
possible  to  say  at  present  whether  the  branchiomeres  and  their  associated 
nerves  correspond  with  one  or  several  of  the  cranial  mesodermic  somites, 
or  whether,  indeed,  any  correspondence  whatever  exists. 

In  early  stages  of  development  a  series  of  constrictions  have  been 
observed  in  the  cranial  portion  of  the  neural  tube  and  have  been  regarded 
as  indicating  a  primitive  segmentation  of  that  structure.  The  neuromeres, 
as  the  intervals  between  successive  constrictions  have  been  termed,  seem 
to  correspond  with  the  cranial  nerves  as  usually  recognized  and  hence 
cannot  be  regarded  as  primitive  segmental  structures.  They  are  more 
probably  secondary  and  due  to  the  arrangement  of  the  neuroblasts  corre- 
sponding to  the  various  nerves. 

The  Development  of  the  Sympathetic  Nervous  System. — 

From  the  embryological  standpoint  the  distinction  which  has  been 
generally  recognized  between  the  sympathetic  and  central  nervous 
systems  does  not  exist,  the  former  having  been  found  to  be  an 
outgrowth  from  the  latter.  This  mode  of  origin  has  been  observed 
with  especial  clearness  in  the  embryos  of  some  of  the  lower  verte- 
brates, in  which  masses  of  cells  have  been  seen  to  separate  from  the 
posterior  root  ganglia  to  form  the  ganglia  of  the  ganglionated  cord 
(Fig.  248).  In  the  mammalia,  including  man,  the  relations  of  the 
two  sets  of  ganglia  to  one  another  is  by  no  means  so  apparent,  since 
the  sympathetic  cells,  instead  of  being  separated  from  the  posterior 
root  ganglion  en  masse,  migrate  from  it  singly  or  in  groups,  and  are 
therefore  less  readily  distinguishable  from  the  surrounding  meso- 
dermal tissues. 

To  understand  the  development  of  the  sympathetic  system  it 
must  be  remembered  that  it  consists  typically  of  three  sets  of  gan- 
glia. One  of  these  is  constituted  by  the  ganglia  of  the  ganglionated 
cord  (Fig.  249,  GC),  the  second  is  represented  by  the  ganglia  of  the 


THE    SYMPATHETIC    SYSTEM 


419 


'"v 


^ 


- 


■ 


Fig.  248. — Transverse  Section  through  an  Embryo  Shark  (Scyllium)  of  ii  mm., 

SHOWING  THE  ORIGIN   OF  A  SYMPATHETIC   GANGLION. 

Ch,  Notochord;  E,  ectoderm;  G,  posterior  root  ganglion;  Gs,  sympathetic  ganglion;  .1/, 

spinal  cord. — (Onodi.) 


420 


THE    SYMPATHETIC    SYSTEM 


prevertebral  plexuses  (PVG),  such  as  the  cardiac,  solar,  hypogas- 
tric, and  pelvic,  while  the  third  or  peripheral  set  {PG)  is  formed  by 
the  cells  which  occur  throughout  the  tissues  of  probably  most  of  the 
visceral  organs,  either  in  small  groups  or  scattered  through  plexuses 
such  as  the  Auerbach  and  Meissner  plexuses  of  the  intestine.  Each 
cell  in  these  various  ganglia  stands  in  direct  contact  with  the  axis- 
cylinder  of  a  cell  situated  in  the  central  nervous  system,  probably  in 
the  lateral  horn  of  the  spinal  cord  or  the  corresponding  region  of  the 
brain,  so  that  each  cell  forms  the  terminal  link  of  a  chain  whose  first 
link  is  a  neurone  belonging  to  the  central  system  (Huber) .    Through- 


Fig.  249. — Diagram  showing  the  Arrangement  of  the  Neurones  of  the  Sympa- 
thetic System. 
The  fibers  from  the  posterior  root  ganglia  are  represented  by  the  broken  black  lines; 
those  from  the  anterior  horn  cells  by  the  solid  black;  the  white  rami  by  red;  and  the 
sympathetic  neurones  by  blue.  DR,  Dorsal  ramus  of  spinal  nerve;  GC,  ganglionated 
cord;  GR,  gray  ramus  communicans;  PG,  peripheral  ganglion;  PVG,  prsevertebral 
ganglion;  VR,  ventral  ramus  of  spinal  nerve;  WR,  white  ramus  communicans. — 
{Adapted  from  Huber.) 

out  the  thoracic  and  upper  lumbar  regions  of  the  body  the  central 
system  neurones  form  distinct  cords  known  as  the  white  rami  com- 
municantes  (Fig.  249,  WR),  which  pass  from  the  spinal  nerves  to  the 
adjacent  ganglia  of  the  ganglionated  cord,  some  of  them  terminat- 
ing around  the  cells  of  these  ganglia,  others  passing  on  to  the  cells  of 
the  prsevertebral  ganglia,  and  others  to  those  of  the  peripheral 
plexuses.  In  the  cervical,  lower  lumbar  and  sacral  regions  white 
rami  are  wanting,  the  central  neurones  in  the  first-named  region 
probably  making  their  way  to  the  sympathetic  cells  by  way  of  the  upper 


THE    SYMPATHETIC    SYSTEM 


421 


thoracic  nerves,  while  in  the  lower  regions  they  may  pass  down  the 
ganglionated  cord  from  higher  regions  or  may  join  the  prevertebral 
and  peripheral  ganglia  directly  without  passing  through  the  proxi- 
mal ganglia.  In  addition  to  these  white  rami,  what  are  known  as 
gray  rami  also  extend  between  the  proximal  ganglia  and  the  spinal 
nerves;  these  are  composed  of  fibers,  arising  from  sympathetic  cells, 


Fig.  250. — Transverse  Section  through  the  Spinal  Cord  of  an  Embryo  of  7  mm. 

c,  Notochord;  g,  posterior  root  ganglion;  m,  spinal  cord;  s,  sympathetic  cell  migrating 

from  the  posterior  root  ganglion;  wr,  white  ramus.- — (His.) 

which  join  the  spinal  nerves  in  order  to  pass  with  them  to  their 
ultimate  distribution. 

The  brief  description  here  given  applies  especially  to  the  sym- 
pathetic system  of  the  neck  and  trunk.  Representatives  of  the 
system  are  also  found  in  the  head,  in  the  form  of  a  series  of  ganglia 
connected  with  the  trigeminal  and  facial  nerves  and  known  as  the 
ciliary,  spheno-palatine,  otic,  and  submaxillary  ganglia;  and,  as  will 


422  THE    SYMPATHETIC    SYSTEM 

be  seen  later,  there  are  probably  some  sympathetic  cells  which  owe 
their  origin  to  the  root  ganglia  of  the  vagus  and  glossopharyngeal 
nerves.  There  is  nothing,  however,  in  the  head  region  corresponding 
to  the  longitudinal  bundles  of  fibers  which  unite  the  various  proximal 
ganglia  of  the  trunk  to  form  the  ganglionated  cord. 

The  first  distinct  indications  of  the  sympathetic  system  are  to  be 
seen  in  a  human  embryo  of  about  7  mm.  As  the  spinal  nerves 
reach  the  level  of  the  dorsal  edge  of  the  body-cavity,  they  branch, 
one  of  the  branches  continuing  ventrally  in  the  body-wall,  while  the 
other  (Fig  250,  wr)  passes  mesially  toward  the  aorta,  some  of  its 
fibers  reaching  that  structure,  while  others  bend  so  as  to  assume  a 
longitudinal  direction.  These  mesial  branches  represent  the  white 
rami  communicantes,  but  as  yet  no  ganglion  cells  can  be  seen  in 
their  course.  The  cells  of  the  posterior  root  ganglia  have  already, 
for  the  most  part,  assumed  their  bipolar  form,  but  among  them  there 
may  still  be  found  a  number  of  cells  in  the  neuroblast  condition,  and 
these  (Fig.  250,  s),  wandering  out  from  the  ganglia,  give  rise  to  a 
column  of  cells  standing  in  relation  to  the  white  rami.  At  first  there 
is  no  indication  of  a  segmental  arrangement  of  the  cells  of  the  column 
(Fig.  251),  but  at  about  the  seventh  week  such  an  arrangement 
makes  its  appearance  in  the  cervical  region,  and  later,  extends 
posteriorly,  until  the  column  assumes  the  form  of  the  ganglionated 
cord. 

This  origin  of  the  ganglionated  cord  from  cells  migrating  out 
from  the  posterior  root  ganglia  has  been  described  by  various 
authors,  but  recently  the  origin  of  the  cells  has  been  carried  a  step 
further  back,  to  the  mantle  layer  of  the  central  nervous  system 
(Kuntz).  Indifferent  cells  and  neuroblasts  are  said  to  wander  out 
from  the  walls  of  the  medullary  canal  by  way  of  both  the  posterior 
and  anterior  nerve  roots  and  it  is  claimed  that  these  are  the  cells  that 
give  rise  to  the  ganglionated  cord  in  the  manner  just  described. 

Before,  however,  the  segmentation  of  the  ganglionated  cord  be- 
comes marked,  thickenings  appear  at  certain  regions  of  the  cell 
column,  and  from  these,  bundles  of  fibers  may  be  seen  extending 
ventrally  toward  the  viscera.     The  thickenings  represent  certain  of 


THE    SYMPATHETIC    SYSTEM 


423 


the  prevertebral  ganglia,  and  later  cells  wander  out  from  them  and 
take  a  position  in  front  of  the  aorta.  In  an  embryo  of  10.2  mm.  two 
ganglionic  masses  (Fig.  251,  pc)  occur  in  the  vicinity  of  the  origin 


Fig.  251. — Reconstruction  of  the  Sympathetic  System  of  an  Embryo  of  10.2  mm. 
am,  Vitelline  artery;  ao,  aorta;  au,  umbilical  artery;  bg,  ganglionic  mass  representing 
the  pelvic  plexus;  d,  intestine;  oe,  oesophagus;  pc,  ganglia  of  the  cceliac  plexus;  ph, 
pharynx;  rv,  right  vagus  nerve;  sp,  splanchnic  nerves;  sy,  ganglionated  cord;  t,  trachea; 
*,  peripheral  sympathetic  ganglia  in  the  walls  of  the  stomach. — (His,  Jr.) 


of  the  vitelline  artery  (am),  one  lying  above  and  the  other  below 
that    vessel;    these    masses  represent  the   ganglia    of    the    cceliac 


424  LITERATURE 

plexus  and  have  separated  somewhat  from  the  ganglionated  cord, 
the  fiber  bundles  which  unite  the  upper  mass  with  the  cord  represent- 
ing the  greater  and  lesser  splanchnic  nerves  (sp),  while  that  connected 
with  the  lower  mass  represents  the  connection  of  the  cord  with  the 
superior  mesenteric  ganglion.  Lower  down,  in  the  neighborhood 
of  the  umbilical  arteries,  is  another  enlargement  of  the  cord  (bg), 
which  probably  represents  the  inferior  mesenteric  and  hypogastric 
ganglia  which  have  not  yet  separated  from  the  cell  column. 

With  the  peripheral  ganglia  the  conditions  are  slightly  different, 
in  that  they  are  formed  very  largely,  if  not  exclusively,  from  cells 
that  migrate  from  the  walls  of  the  hind-brain  by  way  of  the  vagus 
nerves  (Fig.  251).  In  this  way  the  ganglia  of  the  myenteric,  pul- 
monary and  cardiac  plexuses  are  formed,  though  in  the  case  of  the 
last  named  it  is  probable  that  contributions  are  also  received  from 
the  ganglionated  cord. 

The  elongated  courses  of  the  cardiac  sympathetic  and  splanchnic 
nerves  in  the  adult  receive  an  explanation  from  the  recession  of  the  heart 
arid  diaphragm  (see  pp.  239  and  322),  the  latter  process  forcing  down- 
ward the  coeliac  plexus,  which  originally  occupied  a  position  opposite 
the  region  of  the  ganglionated  cord  from  which  the  splanchnic  nerves 
arise. 

As  regards  the  cephalic  sympathetic  ganglia,  the  observations 
of  Remak  on  the  chick  and  Kolliker  on  the  rabbit  show  that  the 
ciliary,  sphenopalatine,  and  otic  ganglia  arise  by  the  separation  of 
cells  from  the  semilunar  (Gasserian)  ganglion,  and  from  their  adult 
relations  it  may  be  supposed  that  the  cells  of  the  submaxillary  and 
sublingual  ganglia  have  similarly  arisen  from  the  geniculate  ganglion 
of  the  facial  nerve.  Evidence  has  also  been  obtained  from  human 
embryos  that  sympathetic  cells  are  derived  from  the  ganglia  of  the 
vagus  and  glossopharyngeal  nerves,  but,  instead  of  forming  distinct 
ganglia  in  the  adult,  these,  in  all  probability,  associate  themselves 
with  the  first  cervical  ganglia  of  the  ganglionated  cord. 

LITERATURE. 

C.  R.  Bardeen:   "The  Growth  and  Histogenesis  of  the  Cerebrospinal  Nerves  in 
Mammals,"  Amer.  Journ.  AnaL,  11,  1903. 


LITERATURE  425 

S.  R.  Cajal:  "Nouvelles  Observations  sur  revolution  des  neuroblasts  avec  quelques 

remarques   sur   l'hypothese  neurogenetique  de  Hensen-Held,"   Anal.    Anzeiger, 

xxxii,  1908. 
A.  F.  Dixon:  "On  the  Development  of  the  Branches  of  the  Fifth  Cranial  Nerve  in 

Man,"  Sclent.  Trans.  Roy.  Dublin  Soc,  Ser.  1,  VI,  1896. 
C.  R.  Essick:  "The  Development  of  the  Nuclei  pontis  and  the  Nucleus  Arcuatus  in 

Man,"  Amer.  Journ.  Anat.,  xiii,  1912. 
E.  Giglio-Tos:  "Sugli  organi  branchiali  e  laterali  di  senso  nell'  uomo  nei  primordi 

del  suo  sviluppo,"  Monit.  Zool.  Ital.,  xill,  1902. 
E.  Giglio-Tos:  "SulP  origine  embrionale  del  nervo  trigemino  nell'  uomo,"  Anat. 

Anzeiger,  xxi,  1902. 
E.  Giglio-Tos:  "Sui  primordi  dello  sviluppo  del  nervo  acustico-faciale  nell'  uomo," 

Anat.  Anzeiger,  xxi,  1902. 
K.    Goldstein:    "Die   erste   Entwicklung   der  grossen   Hirncommissuren   und   die 

'Verwachsung'  von  Thalamus  und  Striatum"   Archiv  jiir  Anat.  und  Physiol., 

Anat.  Abth.,  1903. 
G.  Groenberg:  "Die  Ontogenese  einer  niederen  Saugergehirns  nach  Untersuchungen 

an  Erinaceus  europaeus,"  Zoolog.  Jahrb.  Abth.  f.  Anat.  und  Ontogen.,  xv,  1901. 
I.  Hardesty:  "On  the  Development  and  Nature  of  the  Neuroglia,"  Amer.  Journ 

Anat.,  in,  1904. 
R.  G.  Harrison:  "Further  Experiments  on  the  Development  of  Peripheral  Nerves,' 

Amer.  Journ.  of  Anat.,  v,  1906. 
W.  His:  "Zur  Geschichte  des  menschlichen  Ruckenmarkes  und  der  Nervenwurzeln,' 

Abhandl.  der  konigl.  Sachsischen  Gesellsch.,  Math.-Physik.  Classe,  xiii,  1886. 
W.  His:  "Zur  Geschichte  des  Gehirns  sowie  der  centralen  und  peripherischen  Nerven- 

bahnen  beim  menschlichen  Embryo,"  Abhandl.  der  konigl.  Sachsischen  Gesellsch., 

Math.-Physik.  Classe,  xiv,  1888. 
W.  His:  "Die  Formentwickelung  des  menschlichen  Vorderhirns  vom  Ende  des  ersten 

bis  zum  Beginn  des  dritten  Monats,"  Abhandl.  der  konigl.  Sachsischen  Gesellsch., 

Math.-Physik.  Classe,  xv.  1889. 
W.  His:  "Histogenese  und  Zusammenhang  der  Nervenelemente,"  Archiv  fur  Anat. 

und  Physiol.,  Anat.  Abth.,  Supplement,  1890. 
W.  His:  "Die  Entwickelung  des  menschlichen  Gehirns  wahrend  der  ersten  Monate," 

Leipzig,  1904. 
W.  His,  Jr.:  "Die  Entwickelung  des  Herznervensystem  bei  Wirbelthieren,"  Abhandl. 

der  konigl.  Sachsischen  Gesellsch.,  Math.-Physik.  Classe,  xvni,  1893. 
W.  His,  Jr.:  "Ueber  die  Entwickelung  des  Bauchsympathicus  beim  Huhnchen  und 

Menschen,"  Archiv  filr  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,  1S97. 
C.  J.  Herrick:  "  The  Cranial  and  First  Spinal  Nerves  of  Menidia:  A  Contribution  upon 

the  Nerve  Components  of  the  Bony  Fishes,"  Journ.  of  Comp.  Neurol.,  ix,  1899. 
C.   J.  Herrick:   "The  Cranial  Nerves  and  Cutaneous  Sense-organs  of  the  North 

American  Siluroid  Fishes,"  Journ.  of  Comp.  Neurol.,  xi,  1901. 
G.  C.  Huber:  "Four  Lectures  on  the  Sympathetic  Nervous  System,"  Journ.  of  Comp. 

Neurol.,  vn,  1897. 
A.  Kuntz:  "A  Contribution  to  the  Histogenesis  of  the  Sympathetic  System,"  Anat. 

Record,  in,  1909. 


426  LITERATURE 

A.  Kuntz:  "The  role  of  the  Vagi  in  the  Development  of  the  Sympathetic  Nervous 

System,"  Anat.  Anzeiger,  xxxv,  1909. 
A.  Kuntz:  "The  Development  of  the  Sympathetic  Nervous  System  in  Mammals,' 

Journ.  Compar.  Neurol.,  xx,  1910. 
M.  von  Lenhossek:  "Die  Entwickelung  der  Ganglienanlagen  bei  dem  menschlichen 

Embryo,"  Archiv  filr  Anat.  und  Physiol.,  Anat.  Abth.,  1891. 

F.  Marchand:   "Ueber  die  Entwickelung  des  Balkens  im  menschlichen   Gehirn," 

Archiv  filr  mikrosk.  Anat.,  xxxvn,  1891. 
V.  VON  Mihalkovicz:  " Entwickelungsgeschichte  des  Gehirns,"  Leipzig,  1877. 
A.  D.  Onodi:  "Ueber  die  Entwickelung  des  sympathischen  Nervensystems,"  Archiv 

filr  mikrosk.  Anat.,  xxvn,  1886. 

G.  Retzius:  "Das  Menschenhirn,"  Stockholm,  1896. 

A.    Schaper:    "Die   friihesten   Differenzirungsvorgange   im    Central-nerven-system,' 

Archiv  filr  Entwicklungsmechanik,  v,  1897. 
G.  L.  Streeter:  "  The  Development  of  the  Cranial  and  Spinal  Nerves  in  the  Occipita 

Region  of  the  Human  Embryo,"  Amer.  Journ.  Anat.,  iv,  1904. 
O.  S.  Strong:  "The  Cranial  Nerves  of  Amphibia,"  Journal  of  Morphol.,  x,  1895. 
R.  Wlassak:   "Die  Herkunft  des  Myelins,"  Archiv  filr   Entwicklungsmechanik,  VI 

1898. 
E.  Zuckerkandl:  "Zur  Entwicklung  des  Balkens,"  Arbeiten  aus  neurol.  Inst.  Wien. 

xvii,  1909. 


CHAPTER  XVI. 

THE  DEVELOPMENT  OF  THE  ORGANS  OF 
SPECIAL  SENSE. 

Like  the  cells  of  the  central  nervous  system,  the  sensory  cells 
are  all  of  ectodermal  origin,  and  in  lower  animals,  such  as  the  earth- 
worm, for  instance,  they  retain  their  original  position  in  the  ecto- 
dermal epithelium  throughout  life.  In  the  vertebrates,  however, 
the  majority  of  the  sensory  cells  relinquish  their  superficial  position 
and  sink  more  or  less  deeply  into  the  subjacent  tissues,  being  repre- 
sented by  the  posterior  root  ganglion  cells  and  by  the  sensory  cells 
of  the  special  sense-organs,  and  it  is  only  in  the  olfactory  organ  that 
the  original  condition  is  retained.  Those  cells  which  have  with- 
drawn from  the  surface  receive  stimuli  only  through  overlying  cells, 
and  in  certain  cases  these  transmitting  cells  are  not  specially  differ- 
entiated, the  terminal  branches  of  the  sensory  dendrites  e  ding 
among  ordinary  epithelial  cells  or  in  such  structures  as  the  Pacinian 
bodies  or  the  end-bulbs  of  Krause  situated  beneath  undifferentiated 
epithelium.  In  other  cases,  however,  certain  specially  modified 
superficial  cells  serve  to  transmit  the  stimuli  to  the  peripheral  sensory 
neurones,  forming  such  structures  as  the  hair-cells  of  the  auditory 
epithelium  or  the  gustatory  cells  of  the  taste-buds. 

Thus  three  degrees  of  differentiation  of  the  special  sensory  cells 
may  be  recognized  and  a  classification  of  the  sense-organs  may  be 
made  upon  this  basis.  One  organ,  however,  the  eye,  cannot  be 
brought  into  such  a  classification,  since  its  sensory  cells  present 
certain  developmental  peculiarities  which  distinguish  them  from 
those  of  all  other  sense-organs.  Embryologically  the  retina  is  a 
portion  of  the  central  nervous  system  and  not  a  peripheral  organ, 
and  hence  it  will  be  convenient  to  arrange  the  other  sense-organs 

427 


428  THE    OLFACTORY    ORGANS 

according  to  the  classification  indicated  and  to  discuss  the"  history 
of  the  eye  at  the  close  of  the  chapter. 

The  Development  of  the  Olfactory  Organ. — The  general 
development  of  the  nasal  fossa,  the  epithelium  of  which  contains  the 
olfactory  sense  cells,  has  already  been  described  (pp.  99  and  283), 
as  has  also  the  development  of  the  olfactory  lobes  of  the  brain 
(p.  406),  and  there  remains  for  consideration  here  merely  the  forma- 
tion of  the  olfactory  nerve  and  the  development  of  the  rudimentary 
organ  of  Jacobson. 

The  Olfactory  Nerve. — Very  diverse  results  have  been  obtained  by 
various  observers  of  the  development  of  the  olfactory  nerve,  it  having 
been  held  at  different  times  that  it  was  formed  by  the  outgrowth  of 
fibers  from  the  olfactory  lobes  (Marshall),  from  fibers  which  arise 
partly  from  the  olfactory  lobes  and  partly  from  the  olfactory  epithe- 
lium (Beard),  from  the  cells  of  an  olfactory  ganglion  originally  derived 
from  the  olfactory  epithelium  but  later  separating  from  it  (His), 
and,  finally,  that  it  was  composed  of  the  prolongations  of  certain 
cells  situated  and,  for  the  most  part  at  least,  remaining  permanently 
in  the  olfactory  epithelium  (Disse).  The  most  recent  observations  on 
the  structure  of  the  olfactory  epithelium  and  nerve  indicate  a  greater 
amount  of  probability  in  the  last  result  than  in  the  others,  and  the 
description  which  follows  will  be  based  upon  the  observations  of  His, 
modified  in  conformity  with  the  results  obtained  by  Disse  from  chick 
embryos. 

In  human  embryos  of  the  fourth  week  the  cells  lining  the  upper 
part  of  the  olfactory  pits  show  a  distinction  into  ordinary  epithelial 
and  sensory  cells,  the  latter,  when  fully  formed,  being  elongated 
cells  prolonged  peripherally  into  a  short  but  narrow  process  which 
reaches  the  surface  of  the  epithelium  and  proximally  gives  rise  to 
an  axis-cylinder  process  which  extends  up  toward  and  penetrates  the 
tip  of  the  olfactory  lobe  to  come  into  contact  with  the  dendrites  of 
the  first  central  neurones  of  the  olfactory  tract  (Fig.  252).  These 
cells  constitute  a  neuro-epithelium  and  in  later  stages  of  develop- 
ment retain  their  epithelial  position  for  the  most  part,  a  few  of  them, 
however,  withdrawing  into  the  subjacent  mesenchyme  and  becoming 


THE    OLFACTORY    ORGANS 


429 


bipolar,  their  peripheral  prolongations  ending  freely  among  the  cells 
of  the  olfactory  epithelium.  These  bipolar  cells  resemble  closely 
in  form  and  relations  the  cells  of  the  embryonic  posterior  root  ganglia, 
and  thus  form  an  interesting  transition  between  these  and  the  neuro- 
epithelial cells. 

The  Organ  of  Jacohson. — In  embryos  of  three  or  four  months  a 


Fig.  252. — Diagram  Illustrating  the  Relations  of  the  Fibers  of  the  Olfactory 

Nerve. 

Ep,  Epithelium  of  the  olfactory  pit;  C,  cribiform  plate  of  the'ethmoid,  G,  glomerulus  of 

the  olfactory  bulb;  M,  mitral  cell. — (Van  Gekuchten.)  j 

small  pouch-like  invagination  of  the  epithelium  covering  the  lower 
anterior  portion  of  the  median  septum  of  the  nose  can  readily  be 
seen.  This  becomes  converted  into  a  slender  pouch,  3  to  5  mm.  long, 
ending  blindly  at  its  posterior  extremity  and  opening  at  its  other  end 


430  THE    ORGANS    OF    TASTE 

into  the  nasal  cavity.  Its  lining  epithelium  resembles  that  of  the 
respiratory  portion  of  the  nasal  cavity,  and  there  is  developed  in  the 
connective  tissue  beneath  its  floor  a  slender  plate  of  cartilage,  dis- 
tinct from  that  forming  the  septum  of  the  nose. 

This  organ,  which  may  apparently  undergo  degeneration  in  the 
adult,  and  in  some  cases  completely  disappears,  appears  to  be  the 
representative  of  what  is  known  as  Jacobson's  organ,  a  structure 
which  reaches  a  much  more  extensive  degree  of  development  in  many 
of  the  lower  mammals,  and  in  these  contains  in  its  epithelium  sensory 
cells  whose  axis-cylinder  processes  pass  with  those  of  the  olfactor} 
sense  cells  to  the  olfactory  bulbs.  In  man,  however,  it  seems  to  be  a 
rudimentary  organ,  and  no  satisfactory  explanation  of  its  function 
has  as  yet  been  advanced. 

The  olfactory  neuro-epithelium,  considered  from  a  comparative 
standpoint,  seems  to  have  been  derived  from  the  system  of  lateral 
line  organs  so  highly  developed  in  the  lower  vertebrates  (Kupffer). 
In  higher  forms  the  system,  which  is  cutaneous  in  character,  has 
disappeared  except  in  two  regions  where  it  has  become  highly 
specialized.  In  one  of  these  regions  it  has  given  rise  to  the  olfactory 
sense  cells  and  in  the  other  to  the  similar  cells  of  the  auditory 
apparatus. 

The  Organs  of  Touch  and  Taste. — Little  is  yet  known  con- 
cerning the  development  of  the  various  forms  of  tactile  organs,  which 
belong  to  the  second  class  of  sensory  organs  described  above. 

The  Organs  of  Taste. — The  remaining  organs  of  special  sense 
belong  to  the  third  class,  and  of  these  the  organs  of  taste  present  in 
many  respects  the  simplest  condition.  They  are  developed  prin- 
cipally in  connection  with  the  vallate  and  foliate  papillae  of  the 
tongue,  and  of  the  former  one  of  the  earliest  observed  stages  has 
been  found  in  embryos  of  9  cm.  in  the  form  of  two  ridges  of  epi- 
dermis, lying  toward  the  back  part  of  the  tongue  and  inclined  to  one 
another  in  such  a  manner  as  to  form  a  V  with  the  apex  directed 
backward.  From  these  ridges  solid  downgrowths  of  epidermis 
into  the  subjacent  tissue  occur,  each  downgrowth  having  the  form 
of  a  hollow  truncated  cone  with  its  basal  edge  continuous  with  the 


THE    INTERNAL   EAR  43 1 

superficial  epidermis  (Fig.  253,  A).  In  later  stages  lateral  out- 
growths develop  from  the  deeper  edges  of  the  cone,  and  about  the 
same  time  clefts  appear  in  the  substance  of  the  original  downgrowths 
(Fig.  253,  B)  and,  uniting  together,  finally  open  to  the  surface,  form- 
ing a  trench  surrounding  a  papilla  (Fig.  253,  C).  The  lateral  out- 
growths, which  are  at  first  solid,  also  undergo  an  axial  degeneration 
and  become  converted  into  the  glands  of  Ebner  (b),  which  open  into 
the  trench  near  its  floor.  The  various  papillae  which  occur  in  the 
adult  do  not  develop  simultaneously,  but  their  number  increases 
with  the  age  of  the  fetus,  and  there  is,  moreover,  considerable 
variation  in  the  time  of  their  development. 

The  taste-buds  are  formed  by  a  differentiation  of  the  epithelium 
which  covers  the  papillae,  and  this  differentiation  appears  to  stand 


B  C 

Fig.  253. — Diagrams  Representing  the  Development  of  a  Vallate  Papilla. 
a,  Valley  surrounding  the  papilla;  b,  von  Ebner's  gland. — (Graberg.) 

in  intimate  relation  with  the  penetration  of  fibers  of  the  glosso- 
pharyngeal nerve  into  the  papillae.  The  buds  form  at  various  places 
upon  the  papillae,  and  at  one  period  are  especially  abundant  upon 
their  free  surfaces,  but  in  the  later  weeks  of  intrauterine  life  these 
surface  buds  undergo  degeneration  and  only  those  upon  the  sides 
of  the  trench  persist,  as  a  rule. 

The  foliate  papillae  do  not  seem  to  be  developed  until  some  time 
after  the  circumvallate,  being  entirely  wanting  in  embryos  of  four 
and  a  half  and  five  months,  although  plainly  recognizable  at  the 
seventh  month. 

The  Development  of  the  Ear. — It  is  customary  to  describe  the 
mammalian  ear  as  consisting  of  three  parts,  known  as  the  inner, 
middle,  and  outer  ears,  and  this  division  is,  to  a  certain  extent  at 


432 


THE    INTERNAL   EAR 


least,  confirmed  by  the  embryonic  development.  The  inner  ear, 
which  is  the  sensory  portion  proper,  is  an  ectodermal  structure,  which 
secondarily  becomes  deeply  seated  in  the  mesodermal  tissue  of  the 
head,  while  the  middle  and  outer  ears,  which  provide  the  apparatus 
necessary  for  the  conduction  of  the  sound-waves  to  the  inner  ear, 
are  modified  portions  of  the  anterior  branchial  arches.  It  will  be 
convenient,  accordingly,  in  the  description  of  the  ear,  to  accept 
the  usually  recognized  divisions  and  to  consider  first  of  all  the 
development  of  the  inner  ear,  or,  as  it  is  better  termed,  the  otocyst. 
The  Development  of  the  Otocyst. — In  an  embryo  of  2.4  mm.  a 
pair  of  pits  occur  upon  the  surface  of  the  body  about  opposite  the 
middle  portion  of  the  hind-brain  (Fig.  254,  A).  The  ectoderm 
lining  the  pits  is  somewhat  thicker  than  is  the  neighboring  ectoderm 


a  —  B 

Fig.  254. — Transverse  Section  Passing  through  the  Otocyst  (ot)  of  Embryos  of 
(A)  2.4  mm.  and  (B)  4  mm. — {His.) 

of  the  surface  of  the  body,  and,  from  analogy  with  what  occurs  in 
other  vertebrates,  it  seems  probable  that  the  pits  are  formed  by  the 
invagination  of  localized  thickenings  of  the  ectoderm.  The  mouth 
of  each  pit  gradually  becomes  smaller,  until  finally  the  invagination 
is  converted  into  a  closed  sac  (Fig.  254,  B),  which  separates  from  the 
surface  ectoderm  and  becomes  enclosed  within  the  subjacent  meso- 
derm. This  sac  is  the  otocyst,  and  in  the  stage  just  described, 
found  in  embryos  of  4  mm.,  it  has  an  oval  or  more  or  less  spherical 
form.  Soon,  however,  in  embryos  of  6.9  mm.,  a  prolongation 
arises  from  its  dorsal  portion  and  the  sac  assumes  the  form  shown  in 
Fig.  255,  A;  this  prolongation,  which  is  held  by  some  authors  to  be 
the  remains  of  the  stalk  which  originally  connected  the  otocyst  sac 


THE    INTERNAL   EAR 


433 


with  the  surface  ectoderm,  represents  the  ductus  endolymphaticus , 
and,  increasing  in  length,  it  soon  becomes  a  strong  club-shaped 
process,  projecting  considerably  beyond  the  remaining  portions  of 
the  otocyst  (Fig.  255,  B).  In  embryos  of  about  10.2  mm.  the  sac 
begins  to  show  certain  other  irregularities  of  shape  (Fig.  255,  B,  sc). 
Thus,  about  opposite  the  point  of  origin  of  the  ductus  endolymph- 
aticus three  folds  make  their  appearance,  representing  the  semi- 


-rsc 


Fig.  255. — Reconstruction  of  the  Otocysts  of  Embryo  of  (A)  6.9  mm.  and  (B) 

10.2  MM. 

de,  Endolymphatic  duct;  gc,  ganglion  cochleare;  gg,  ganglion  geniculatum;  gv, 

ganglion  vestibulare;  sc,  lateral  semicircular  duct. — (His,  Jr.) 

circular  ducts,  and  as  they  increase  in  size  the  opposite  walls  of  the 
central  portion  of  each  fold  come  together,  fuse,  and  finally  become 
absorbed,  leaving  the  free  edge  of  the  fold  as  a  crescentic  canal,  at 
one  end  of  which  an  enlargement  appears  to  form  the  ampulla.  The 
transformation  of  the  folds  into  canals  takes  place  somewhat  earlier 
in  the  cases  of  the  two  vertical  than  in  that  of  the  horizontal  duct,  as 
28 


434 


THE    INTERNAL   EAR 


may  be  seen  from  Fig.  256,  which  represents  the  condition  occurring 

in  an  embryo  of  13.5  mm. 

A  short  distance  below  the  level  at  which  the  canals  communicate 

with  the  remaining  portion  of  the  otocyst  a  constriction  appears, 

indicating  a  separation  of  the  otocyst 
into  a  more  dorsal  portion  and  a  more 
ventral  one.  Later,  the  latter  begins 
to  be  prolonged  into  a  flattened  canal 
which,  as  it  elongates,  becomes  coiled 
upon  itself  and  also  becomes  separated 
by  a  constriction  from  the  remaining 
portion  of  the  otocyst  (Fig.  257). 
This  canal  is  the  ductus  cochlearis 
(scala  media  of  the  cochlea),  and  the 
remaining  portion  of  the  otocyst  sub- 
sequently becomes  divided  by  a  con- 
striction into  the  utriculus,  with  which 
the  semicircular  ducts  are  connected, 
and  the  sacculus.  The  constriction 
which  separates  the  cochlear  duct  from 
the  sacculus  becomes  the  ductus  re- 
uniens,  while  that  between  the  utri- 
culus and  sacculus  is  converted  into 
a  narrow  canal  with  which  the  ductus 
endolymphaticus  connects,  and  hence 
it  is  that,  in  the  adult,  the  connection 
between  these  two  portions  of  H  the 
otocyst  seems  to  be  formed  by  the 
ductus  dividing  proximally  into  two 
limbs,  one  of  which  is  connected  with 

the  utricle  and  the  other  with  the  saccule. 

When  first  observed  in  the  human  embryo  the  auditory  ganglion 

is  closely  associated  with  the  geniculate  ganglion  of  the  seventh 

nerve  (Fig.  255,  B),  the  two,  usually  spoken  of  as  the  acustico-facialis 

ganglion,  forming  a  mass  of  cells  lying  in  close  contact  with  the 


Fig.  256. — Reconstruction  of 
the  Otocyst  of  an   Embryo  of 

I3.5   MM. 

co,  Cochlea;  de,  endolymphatic 
duct;.sc,  semicircular  duct. — (His 
Jr.) 


THE    INTERNAL  EAR 


435 


anterior  wall  of  the  otocyst.  The  origin  of  the  ganglionic  mass  has 
not  yet  been  traced  in  the  mammalia,  but  it  has  been  observed  that 
in  cow  embryos  the  geniculate  ganglion  is  connected  with  the  ecto- 
derm at  the  dorsal  end  of  the  first  branchial  cleft  (Froriep),  and  it 
may  perhaps  be  regarded  as  one  of  the  epibranchial  placodes  (see  p. 
417),  and  in  the  lower  vertebrates  a  union  of  the  ganglion  with  a 
suprabranchial   placode  has  been  observed  (Kupffer),  this  union 


Fig.  257. — Reconstruction  of  the  Otocyst  of  an  Embryo  of  20  mm.,  front  view. 
cc,  Common  limb  of  superior  and  posterior  semicircular  ducts;  eg,  cochlear  ganglion; 
co,  cochlea;  de,  endolymphatic  duct;  s,  sacculus;  sdl,  sdp,  and  sdsx  lateral,  posterior  and 
superior  semicircular  ducts;  u,  utriculus;  vg,  vestibular  ganglion. — {Streeter.) 

indicating  the  origin  of  the  auditory  ganglion  from  one  or  more  of 
the  ganglia  of  the  lateral  line  system. 

At  an  early  stage  in  the  human  embryo  the  auditory  ganglion 
shows  indications  of  a  division  into  two  portions,  a  more  dorsal  one, 
which  represents  the  future  ganglion  vestibular  e,  and  a  ventral  one, 
the  ganglion  cochleare.  The  ganglion  cells  become  bipolar,  in  which 
condition  they  remain  throughout  life,  never  reaching  the  T-shaped 
condition  found  in  most  of  the  other  peripheral  cerebro-spinal  gang- 
lia.    One  of  the  prolongations  of  each  cell  is  directed  centrally  to 


43  6 


THE    INTERNAL   EAR 


form  a  fiber  of  the  auditory  nerve,  while  the  other  penetrates  the  wall 
of  the  otocyst  to  enter  into  relations  with  certain  specially  modified 
cells  which  differentiate  from  its  lining  epithelium. 

■In  the  earliest  stages  the  ectodermal  lining  of  the  otocyst  is 
formed  of  similar  columnar  cells,  but  later  over  the  greater  part  of 
the  surface  the  cells  flatten  down,  only  a  few,  aggregated  together  to 


Fig.  258. — The  Right  Internal  Ear  of  an  Embryo  of  Six  Months. 
ca,  ce,  and  cp,  Superior,  lateral,  and  posterior  semicircular  ducts;  cr,  crista  acustica; 
de,  endolymphatic  duct;  Is,  spiral  ligament;  mb,  basilar  membrane;  ms  and  tnu,  macula 
acustica  sacculi  and  utriculi;  rb,  basilar  branches  of  the  cochlear  nerve. — (Retzius.) 

form  patches,  retaining  the  high  columnar  form  and  developing  hair- 
like processes  upon  their  free  surfaces.  These  are  the  sensory  cells 
of  the  ear.  In  the  human  ear  there  are  in  all  six  patches  of  these 
sensory  cells,  an  elongated  patch  {crista  ampullaris)  in  the  ampulla  of 
each  semicircular  canal  (Fig.  258,  cr),  a  round  patch   {macula  acus- 


THE    INTERNAL   EAR  437 

tica,  mii)  in  the  utriculus  and  another  (ms)  in  the  sacculus,  and, 
finally,  an  elongated  patch  which  extends  the  entire  length  of  the 
scala  media  of  the  cochlea  and  forms  the  sensory  cells  of  the  spiral 
organ  of  Corti.  The  cells  of  this  last  patch  are  connected  with  the 
fibers  from  the  cochlear  ganglion,  while  those  of  the  vestibular 
ganglion  pass  to  the  cristas  and  maculae. 

In  connection  with  the  spiral  organ  certain  adjacent  cells  also 
retain   their   columnar  form   and  undergo   various  modifications, 


ftfJ^\-\' 


<&WB 


%^, 


l&i^Spfc 


Fig.  259. — Section  of  the  Cochlear  Duct  of  a  Rabbit  Embryo  of  55  mm. 

a,   Mesenchyme;  b  to  e,  epithelium  of  cochlear  duct;  M.t,  membrana  tectoria;  V.s.p, 

vein;  1  to  7,  spiral  organ  of  Corti. — (Baginsky.) 

giving  rise  to  a  rather  complicated  structure  whose  development  has 
been  traced  in  the  rabbit.  Along  the  whole  length  of  the  cochlear 
duct  the  cells  resting  upon  that  half  of  the  basilar  membrane  which  is 
nearest  the  axis  of  the  cochlea,  and  may  be  termed  the  inner  half, 
retain  their  columnar  shape,  forming  two  ridges  projecting  slightly 
into  the  cavity  of  the  scala  (Fig.  259).  The  cells  of  the  inner  ridge, 
much  the  larger  of  the  two,  give  rise  to  the  membrana  tectoria, 


438  THE    INTERNAL   EAR 

either  as  a  cuticular  secretion  or  by  the  artificial  adhesion  of  long 
hair-like  processes  which  project  from  their  free  surfaces  (Ayers). 
The  cells  of  the  outer  ridge  are  arranged  in  six  longitudinal  rows 
(Fig.  259,  1-6);  those  of  the  innermost  row  (1)  develop  hairs  upon 
their  free  surfaces  and  form  the  inner  hair  cells,  those  of  the  next  two 

rows  (2  and  3)  gradually  be- 
come transformed  on  their  ad- 
jacent   surfaces    into     chitinous 

p /''-   /^  substance  and  form  the  rods  of 

Corti,  while  the  three  outer  rows 


;       (4  to  6)  develop  into  the  outer 
e-^~~  __    hair  cells.     It  is   in   connection 

with  the  hair  cells  that  the  per- 
ipheral prolongations  of  the  cells 
of    the    cochlear    ganglion    ter- 
™JS=  d^t"1™    mi«ate,  and  since  these  hair  cells 

Rabbit  Embryo  of  Twenty-four  Days,     are  arranged  in  rows   extending 

c,  Periotic  cartilage;  ep,    fibrous  mem-  .1      pntjrp  ]er,a\h   of  the  cochlear 

brane  beneath  the  epithelium  of  the  canal;  me  ermre  lengin  01  me  COCIliear 

p,  perichondrium;  s,  spongy  tissue.— (Von  duct,   the  ganglion  also  IS  drawn 
Kolliker.)  .  1        ..  . 

out  into  a  spiral  following  the 
coils  of  the  cochlea,  and  hence  is  sometimes  termed  the  spiral 
ganglion. 

While  the  various  changes  described  above  have  been  taking 
place  in  the  otocyst,  the  mesoderm  surrounding  it  has  also  been 
undergoing  development.  At  first  this  tissue  is  quite  uniform  in 
character,  but  later  the  cells  immediately  surrounding  the  otocyst 
condense  to  give  rise  to  a  fibrous  layer  (Fig.  260,  ep),  while  more 
peripherally  they  become  more  loosely  arranged  and  form  a  some- 
what gelatinous  layer  (s) ,  and  still  more  peripherally  a  second  fibrous 
layer  is  differentiated  and  the  remainder  of  the  tissue  assumes  a 
character  which  indicates  an  approaching  conversion  into  cartilage. 
The  further  history  of  these  various  layers  is  as  follows:  The  inner 
fibrous  layer  gives  rise  to  the  connective-tissue  wall  which  supports 
the  ectodermal  lining  of  the  various  portions  of  the  otocyst;  the 
gelatinous  layer  undergoes  a  degeneration  to  form  a  lymph-like 


THE    INTERNAL   EAR 


439 


fluid  known  as  the  perilymph,  the  space  occupied  by  the  fluid  being 
the  perilymphatic  space;  the  outer  fibrous  layer  becomes  peri- 
chondrium and  later  periosteum;  and  the  precartilage  undergoes 
chondrification  and  later  ossifies  to  form  the  petrous  portion  of  the 
temporal  bone. 

The  gelatinous  layer  completely  surrounds  most  of  the  otocyst 
structures,  which  thus  come  to  lie  free  in  the  perilymphatic  space, 
but  in  the  cochlear  region  the  conditions  are  somewhat  different. 
In  this  region  the  gelatinous  layer  is  interrupted  along  two  lines, 


Fig.  261. — Diagrammatic  Transverse  Section  through  a  Coil  of  the  Cochlea 

showing  the  relation  of  the  scal^e. 
c,  Organ  of  Corti;  co,  ganglion  cochleare;  Is,  lamina  spiralis;  SAT,  cochlear  duct;  ST, 
scala  tympani;  SV,  scala  vestibuli. — (From  Gerlach.) 

an  outer  broad  one  where  the  connective-tissue  wall  of  the  cochlear 
duct  is  directly  continuous  with  the  perichondrium  layer,  and  an 
inner  narrow  one,  along  which  a  similar  fusion  takes  place  with  the 
perichondrium  of  a  shelf-like  process  of  the  cartilage,  which  later 
ossifies  to  form  the  lamina  spiralis.  Consequently  throughout  the 
cochlear  region  the  perilymphatic  space  is  divided  into  two  compart- 
ments which  communicate  at  the  apex  of  the  cochlea,  while  below 
one,  known  as  the  scala  vestibuli,  communicates  with  the  space 


440  THE    MIDDLE    EAR 

surrounding  the  saccule  and  utricle,  and  the  other,  the  scala  tympani, 
abuts  upon  a  membrane  which  separates  it  from  the  cavity  of  the 
middle  ear  and  represents  a  portion  of  the  outer  wall  of  the  petrous 
bone  where  chondrification  and  ossification  have  failed  to  occur. 
This  membrane  closes  what  appears  in  the  dried  skull  to  be  an 
opening  in  the  inner  wall  of  the  middle  ear,  known  as  the  fenestra 
cochlea  {rotunda) ;  another  similar  opening,  also  closed  by  membrane 
in  the  fresh  skull,  occurs  in  the  bony  wall  opposite  the  utricular 
portion  of  the  otocyst  and  is  known  as  the  fenestra  vestibuli  (ovalis) . 

The  Development  of  the  Middle  Ear. — The  middle  ear  develops 
from  the  upper  part  of  the  pharyngeal  groove  which  represents  the 
endodermal  portion  of  the  first  branchial  cleft.  This  becomes 
prolonged  dorsally  and  at  its  dorsal  end  enlarges  to  form  the  tym- 
panic cavity,  while  the  narrower  portion  intervening  between  this 
and  the  pharyngeal  cavity  represents  the  tuba  auditiva  (Eustachian 
tube). 

To  correctly  understand  the  development  of  the  tympanic 
cavity  it  is  necessary  to  recall  the  structures  which  form  its  bound- 
aries. Anteriorly  to  the  upper  end  of  the  first  branchial  pouch 
there  is  the  upper  end  of  the  first  arch,  and  behind  it  the  correspond- 
ing part  of  the  second  arch,  the  two  fusing  together  dorsal  to  the 
tympanic  cavity  and  forming  its  roof.  Internally  the  cavity  is 
bounded  by  the  outer  wall  of  the  cartilaginous  investment  of  the 
otocyst,  while  externally  it  is  separated  from  the  upper  part  of  the 
ectodermal  groove  of  the  first  branchial  cleft  by  the  thin  membrane 
which  forms  the  floor  of  the  groove. 

It  has  been  seen  in  an  earlier  chapter  that  the  axial  mesoderm 
of  each  branchial  arch  gives  rise  to  skeletal  structures  and  muscles. 
The  axial  cartilage  of  the  ventral  portion  of  the  first  arch  is  what  is 
known  as  Meckel's  cartilage,  but  in  that  portion  of  the  arch  which 
forms  the  roof  and  anterior  wall  of  the  tympanic  cavity,  the  cartilage 
becomes  constricted  to  form  two  masses  which  later  ossify  to  form  the 
malleus  and  incus  (Fig.  262,  m  and  i),  while  the  muscular  tissue  of 
this  dorsal  portion  of  the  arch  gives  rise  to  the  tensor  tympani.  Simi- 
larly,  in  the  case  of  the  second  arch  there  is  to  be  found,  dorsal  to 


THE    MIDDLE    EAR  44 1 

the  extremity  of  the  cartilage  which  forms  the  styloid  process  of  the 
adult,  a  narrow  plate  of  cartilage  which  forms  an  investment  for 
the  facial  nerve  (Fig.  262,  VII),  and  dorsal  to  this  a  ring  of  cartilage 
(st)  which  surrounds  a  small  stapedial  artery  and  represents  the 
stapes. 

It  has  been  found  that  in  the  rabbit  the  mass  of  cells  from  which 
the  stapes  is  formed  is  at  its  first  appearance  quite  independent  of 
the  second  branchial  arch  (Fuchs),  and  it  has  been  held  to  be  a 


Fig.  262. — Semi-diagrammatic  Viewof  the  Auditory  Ossicles  of  an  Embryo  of 

Six  Weeks. 
i,  Incus;  j,  jugular  vein;  m,  malleus;  mc,  Meckel's  cartilage;  oc,  capsule  of  otocyst; 
R,  cartilage  of  the  second  branchial  arch;  st,  stapes;  VII,  facial  nerve. — (Siebenmann.) 

derivative  of  the  mesenchyme  from  which  the  periotic  capsule  is 
formed.  In  later  stages,  however,  it  becomes  connected  with  the 
cartilage  of  the  second  branchial  arch,  as  shown  in  Fig.  262,  and 
it  is  a  question  whether  this  connection,  which  is  transitory,  does 
not  really  indicate  the  phylogenetic  origin  of  the  ossicle  from  the 
second  arch  cartilage,  its  appearance  as  an  independent  structure 
being  a  secondary  ontogenetic  phenomenon.  However  that  may 
be,  the  stapedial  artery  disappears  in  later  stages  and  the  stapedius 


442 


THE    MIDDLE    EAR 


P- 


\M 


T 


mi 


muscle,  derived  from  the  musculature  of  the  second  branchial  arch 
and  therefore  supplied  by  the  facial  nerve,  becomes  attached  to  the 
ossicle. 

The  three  ossicles  at  first  lie  embedded  in  the  mesenchyme 
forming  the  roof  of  the  primitive  tympanic  cavity,  as  does  also  the 

chorda  tympani,  a  branch  of  the 
seventh  nerve,  as  it  passes  into  the 
substance  of  the  first  arch  on  the  way 
to  its  destination.  The  mesenchyme 
in  which  these  various  structures  are 
embedded  is  rather  voluminous  (Fig. 
264),  and  after  the  end  of  the  seventh 
month  becomes  converted  into  a  pecu- 
liar spongy  tissue,  which,  toward  the 
end  of  fetal  life,  gradually  degener- 
ates, the  tympanic  cavity  at  the  same 
time  expanding  and  wrapping  itself 
around  the  ossicles  and  the  muscles 
attached  to  them  (Fig.  263).  The 
bones  and  their  muscles,  consequently, 
while  appearing  in  the  adult  to  tra- 
verse the  tympanic  cavity,  are  really 
completely  enclosed  within  a  layer  of 
epithelium  continuous  with  that  lining 
the  wall  of  the  cavity,  while  the 
handle  of  the  malleus  and  the  chorda 
tympani  lie  between  the  epithelium  of 
the  outer  wall  of  the  cavity  and  the 
fibrous  mesoderm  which  forms  the 
tympanic  membrane. 
The  extension  of  the  tympanic  cavity  does  not,  however,  cease 
with  its  replacement  of  the  degenerated  spongy  mesenchyme,  but 
toward  the  end  of  fetal  life  it  begins  to  invade  the  substance  of  the 
temporal  bone  by  a  process  similar  to  that  which  produces  the 
ethmoidal  cells  and  the  other  osseous  sinuses  in  connection  with  the 


m 


•M 


Fig.  263. — Diagrams  Illus- 
trating the  Mode  of  Exten- 
sion of  the  Tympanic  Cavity 
Around  the  Auditory  Ossicles. 

M,  Malleus;  m,  spongy  mesen- 
chyme; p,  surface  of  the  periotic 
capsule;  T,  tympanic  cavity. 
The  broken  line  represents  the 
epithelial  lining  of  the  tympanic 
cavity. 


THE    EXTERNAL  EAR  443 

nasal  cavities  (see  p.  175).  This  process  continues  for  some  years 
after  birth  and  results  in  the  formation  in  the  mastoid  portion  of  the 
bone  of  the  so-called  mastoid  cells,  which  communicate  with  the 
tympanic  cavity  and  have  an  epithelial  lining  continuous  with  that 
of  the  cavity. 

The  lower  portion  of  the  diverticulum  from  the  first  pharyngeal 
groove  which  gives  rise  to  the  tympanic  cavity  becomes  converted 
into  the  Eustachian  tube.  During  development  the  lumen  of  the 
tube  disappears  for  a  time,  probably  owing  to  a  proliferation  of  its 
lining  epithelium,  but  it  is  re-established  before  birth. 

In  the  account  of  the  development  of  the  ear-bones  given  above  it  is 
held  that  the  malleus  and  incus  are  derivatives  of  the  first  branchial 
(mandibular)  arch  and  the  stapes  probably  of  the  second.  This  view 
represents  the  general  consensus  of  recent  workers  on  the  difficult  ques- 
tion of  the  origin  of  these  bones,  but  it  should  be  mentioned  that  nearly 
all  possible  modes  of  origin  have  been  at  one  time  or  other  suggested. 
The  malleus  has  very  generally  been  accepted  as  coming  from  the  first 
arch,  and  the  same  is  true  of  the  incus,  although  some  earlier  authors  have 
assigned  it  to  the  second  arch.  But  with  regard  to  the  stapes  the  opin- 
ions have  been  very  varied.  It  has  been  held  to  be  derived  from  the  first 
arch,  from  the  second  arch,  from  neither  one  nor  the  other,  but  from  the 
cartilaginous  investment  of  the  otocyst,  or,  finally,  it  has  been  held  to  have 
a  compound  origin,  its  arch  being  a  product  of  the  second  arch  while  its 
basal  plate  was  a  part  of  the  otocyst  investment. 

The  Development  of  the  Tympanic  Membrane  and  of  the  Outer 
Ear. — Just  as  the  tympanic  cavity  is  formed  from  the  endodermal 
groove  of  the  first  branchial  cleft,  so  the  outer  ear  owes  its  origin  to 
the  ectodermal  groove  of  the  same  cleft  and  to  the  neighboring  arches. 
The  dorsal  and  most  ventral  portions  of  the  groove  flatten  out  and 
disappear,  but  the  median  portion  deepens  to  form,  at  about  the 
end  of  the  second  month,  a  funnel-shaped  cavity  which  corresponds 
to  the  outer  portion  of  the  external  auditory  meatus.  From  the 
inner  end  of  this  a  solid  ingrowth  of  ectoderm  takes  place,  and  this, 
enlarging  at  its  inner  end  to  form  a  disk-like  mass,  comes  into  rela- 
tion with  the  gelatinous  mesoderm  which  surrounds  the  malleus  and 
chorda  tympani.  At  about  the  seventh  month  a  split  occurs  in  the 
disk-like  mass  (Fig.  264),  separating  it  into  an  outer  and  an  inner 


444 


THE    EXTERNAL    EAR 


layer,  the  latter  of  which  becomes  the  outer  epithelium  of  the 
tympanic  membrane.  Later,  the  split  extends  outward  in  the 
substance  of  the  ectodermal  ingrowth  and  eventually  unites  with 
the  funnel-shaped  cavity  to  complete  the  external  meatus. 

The  tympanic  membrane  is  formed  in  considerable  part  from 


me' 


Fig.  264. — Horizontal  Section  Passing  through  the  Dorsal  Wall  of  the 

External  Auditory  Meatus  in  an  Embryo  of  4.5  cm. 

c,  Cochlea;  de,  endolymphatic  duct;  i,  incus;  Is,  transverse  sinus;  m,  malleus;  me, 

meatus  auditorius  externus;  me' ,  cavity  of  the  meatus;  s,  sacculus;  sc,  lateral  semicircular 

canal;  sc',  posterior  semicircular  canal;  st,  stapes;  t,  tympanic  cavity;  u,  utriculus;  7, 

facial  nerve. — {Siebenmann.) 

the  substance  of  the  first  branchial  arch,  the  area  in  which  it  occurs 
not  being  primarily  part  of  the  wall  of  the  tympanic  cavity,  but  being 
brought  into  it  secondarily  by  the  expansion  of  the  cavity.  The 
membrane  itself  is  mesodermal  in  origin  and  is  lined  on  its  outer 


THE    EXTERNAL   EAR 


445 


surface   by   an   ectodermal  and  on  the  inner  by  an  endodermal 
epithelium. 

The  auricle  {pinna)  owes  its  origin  to  the  portions  of  the  first  and 
second  arches  which  bound  the  entrance  of  the  external  meatus. 
Upon  the  posterior  edge  of  the  first  arch  there  appear  about  the 
end  of  the  fourth  week  two  transverse  furrows  which  mark  off  three 
tubercles  (Fig.  258,  A,  1-3)  and  on  the  anterior  edge  of  the  second 


c 


/  F 


Fig.  265. — Stages  in  the  Development  of  the  Auricle. 

A,  Embryo  of  n  mm.;  B,  of  13.6  mm.;  C,  of  15  mm.;  D,  at  the  beginning  of  the  third 

month;  E,  fetus  of  8.5  cm.;  F,  fetus  at  term. — (His.) 

arch  a  corresponding  number  of  tubercles  (4-6)  is  formed,  while,  in 
addition,  a  longitudinal  furrow,  running  down  the  middle  of  the 
arch,  marks  ofT  a  ridge  (c)  lying  posterior  to  the  tubercles.  From 
these  six  tubercles  and  the  ridge  are  developed  the  various  parts  of 
the  auricle,  as  may  be  seen  from  Fig.  265  which  represents  the 


446  THE    EYE 

transformation  as  described  by  His.  According  to  this,  the  most 
ventral  tubercle  of  the  first  arch  (i)  gives  rise  to  the  tragus,  and  the 
middle  one  (5)  of  the  second  arch  furnishes  the  antitragus.  The 
middle  and  dorsal  tubercles  of  the  first  arch  (2  and  3)  unite  with  the 
ridge  (c)  to  produce  the  helix,  while  from  the  dorsal  tubercle  of  the 
second  arch  (4)  is  produced  the  anthelix  and  from  the  ventral  one  (6) 
the  lobule.  More  recent  observations,  however,  seem  to  indicate 
that  the  lobule  is  an  accessory  structure  unrelated  to  the  tubercles 
and  that  the  sixth  tubercle  gives  rise  to  the  antitragus,  while  the 
fifth  is  either  included  in  the  anthelix  or  else  disappears.  It  is 
noteworthy  that  up  to  about  the  third  month  of  development  the 
upper  and  posterior  portion  of  the  helix  is  bent  forward  so  as  to 
conceal  the  anthelix  (Fig.  265,  D);  it  is  at  just  about  a  corresponding 
stage  that  the  pointed  form  of  the  ear  seen  in  the  lower  mammals 
makes  its  appearance,  and  it  is  evident  that,  were  it  not  for  the  for- 
ward bending,  the  human  ear  would  also  be  assuming  at  this  stage 
a  more  or  less  pointed  form.  Indeed,  there  is  usually  to  be  found 
upon  the  incurved  edge  of  the  helix,  some  distance  below  the  upper 
border  of  the  auricle,  a  more  or  less  distinct  tubercle,  known  as 
Darwin's  tubercle,  which  seems  to  represent  the  point  of  the  typical 
mammalian  ear,  and  is,  accordingly,  the  morphological  apex  of  the 
pinna. 

There  seems  to  be  little  room  for  doubt  that  the  otocyst  belongs 
primarily  to  the  system  of  lateral  line  sense-organs,  but  a  discussion  of  this 
interesting  question  would  necessitate  a  consideration  of  details  concern- 
ing the  development  of  the  lower  vertebrates  which  would  be  foreign  to 
the  general  plan  of  this  book.  It  may  be  recalled,  however,  that  the 
analysis  of  the  components  of  the  cranial  nerves  described  on  page  415 
refers  the  auditory  nerve  to  the  lateral  line  system. 

The  Development  of  the  Eye. — The  first  indications  of  the 
development  of  the  eye  are  to  be  found  in  a  pair  of  hollow  out- 
growths from  the  side  of  the  first  primary  brain  vesicle,  at  a  level 
which  corresponds  to  the  junction  of  the  dorsal  and  ventral  zones. 
Each  evagination  is  directed  at  first  upward  and  backward,  and, 
enlarging  at  its  extremity,  it  soon  shows  a  differentiation  into  a 


THE    EYE  447 

terminal  bulb  and  a  stalk  connecting  the  bulb  with  the  brain  (Fig. 
232).  At  an  early  stage  the  bulb  comes  into  apposition  with  the 
ectoderm  of  the  side  of  the  head,  and  this,  over  the  area  of  con- 


Fig.  266. — Early  Stages  in  the  Development  of  the  Lens  in  a  Rabbit  Embryo. 
The  nucleated  layer  to  the  left  is  the  ectoderm  and  the  thicker  lens  epithelium, 
beneath  which  is  the  outer  wall  of  the  optic  evagination;  above  and  below  between  the 
two  is  mesenchyme. — (Rabl.) 

tact,  becomes  thickened  and  then  depressed  to  form  the  beginning 
of  the  future  lens  (Fig.  266). 

As  the  result  of  the  depression  of  the  lens  ectoderm,  the  outer  wall 


448 


THE    EYE 


of  the  optic  bulb  becomes  pushed  inward  toward  the  inner  wall,  and 
this  invagination  continuing  until  the  two  walls  come  into  contact, 
the  bulb  is  transformed  into  a  double-walled  cup,  the  optic  cup,  in 
the  mouth  of  which  lies  the  lens  (Fig.  268).  The  cup  is  not  perfect, 
however,  since  the  invagination  affects  not  only  the  optic  bulb,  but 
also  extends  medially  on  the  posterior  surface  of  the  stalk,  forming 
upon  this  a  longitudinal  groove  and  producing  a  defect  of  the  ventral 
wall  of  the  cup,  known  as  the  chorioidal  fissure  (Fig.  267).  The 
groove  and  fissure  become  occupied  by  mesodermal  tissue,  and  in 
this,  at  about  the  fifth  week,  a  blood-vessel  develops  which  traverses 


Fig.  267. — Reconstruction  of  the  Brain  of  an  Embryo  of  Four  Weeks,  showing 
the  Chorioid  Fissure. — (His.) 

the  cavity  of  the  cup  to  reach  the  lens  and  is  known  as  the  arteria 
hyaloidea. 

In  the  meantime  further  changes  have  been  taking  place  in  the 
lens.  The  ectodermal  depression  which  represents  it  gradually 
deepens  to  form  a  cup,  the  lips  of  which  approximate  and  finally 
meet,  so  that  the  cup  is  converted  into  a  vesicle  which  finally  sepa- 
rates completely  from  the  ectoderm  (Fig.  268),  much  in  the  same 
way  as  the  otocyst  does.  As  the  lens  vesicle  is  constricted  off,  the 
surrounding  mesodermal  tissue  grows  in  to  form  a  layer  between 
it  and  the  overlying  ectoderm,  and  a  split  appearing  in  the  layer 


THE    EYE 


449 


divides  it  into  an  outer  thicker  portion,  which  represents  the  cornea, 
and  an  inner  thinner  portion,  which  covers  the  outer  surface  of  the 
lens  and  becomes  highly  vascular.  The  cavity  between  these  two 
portions  represents  the  anterior  chamber  of  the  eye.  The  cavity  of 
the  optic  cup  has  also  become  filled  by  a  peculiar  tissue  which  repre- 
sents the  vitreous  humor,  while  the  mesodermal  tissue  surrounding 


Fig.  268. — Horizontal  Section  through  the  Eye  of  an  Embryo  Pig  of  7  mm. 
Br,  Diencephalon;  Ec,  ectoderm;  I,  lens;  P,  pigment,  and  R,  retinal  layers  of  the  retina. 


the  cup  condenses  to  form  a  strong  investment  for  it,  which  is  ex- 
ternally continuous  with  the  cornea,  and  at  about  the  sixth  week 
shows  a  differentiation  into  an  inner  vascular  layer,  the  chorioid  coat, 
and  an  outer  denser  one,  which  becomes  the  sclerotic  coat. 

The  various  processes  resulting  in  the  formation  of  the  eye, 
29 


450  THE    LENS 

which  have  thus  been  rapidly  sketched,  may  now  be  considered  in 
greater  detail. 

The  Development  of  the  Lens. — When  the  lens  vesicle  is  complete, 
it  forms  a  more  or  less  spherical  sac  lying  beneath  the  superficial 
ectoderm  and  containing  in  its  cavity  a  few  cells,  either  scattered 
or  in  groups  (Fig.  268).  These  cells,  which  have  wandered  into 
the  cavity  of  the  vesicle  from  its  walls,  take  no  part  in  the  further 
development  of  the  lens,  but  early  undergo  complete  degeneration, 
and  the  first  change  which  is  concerned  with  the  actual  formation 
of  the  lens  is  an  increase  in  the  height  of  the  cells  forming  its  inner 
wall  and  a  thinning  out  of  its  outer  wall  (Fig.  269,  A).  These 
changes  continuing,  the  outer  half  of  the  vesicle  becomes  converted 
into  a  single  layer  of  somewhat  flat  cells  which  persist  in  the  adult 
condition  to  form  the  anterior  epithelium  of  the  lens,  while  the  cells  of 
the  posterior  wall  form  a  marked  projection  into  the  cavity  of  the 
vesicle  and  eventually  completely  obliterate  it,  coming  into  contact 
with  the  inner  surface  of  the  anterior  epithelium  (Fig.  269,  B). 

These  posterior  elongated  cells  form,  then,  the  principal  mass 
of  the  lens,  and  constitute  what  are  known  as  the  lens  fibers.  At 
first  those  situated  at  the  center  of  the  posterior  wall  are  the  longest, 
the  more  peripheral  ones  gradually  diminishing  in  length  until  at 
the  equator  of  the  lens  they  become  continuous  with  and  pass  into  the 
anterior  epithelium.  As  the  lens  increases  in  size,  however,  the 
most  centrally  situated  cells  fail  to  elongate  as  rapidly  as  the  more 
peripheral  ones  and  are  pushed  in  toward  the  center  of  the  lens,  the 
more  peripheral  fibers  meeting  below  them  along  a  line  passing 
across  the  inner  surface  of  the  lens.  The  disparity  of  growth  con- 
tinuing, a  similar  sutural  line  appears  on  the  outer  surface  beneath 
the  anterior  epithelium,  and  the  fibers  become  arranged  in  concen- 
tric layers  around  a  central  core  composed  of  the  shorter  fibers. 
In  the  human  eye  the  line  of  suture  of  the  peripheral  fibers  becomes 
bent  so  as  to  consist  of  two  limbs  which  meet  at  an  angle,  and  from 
the  angle  a  new  sutural  line  develops  during  embryonic  life,  so  that 
the  suture  assumes  the  form  of  a  three-rayed  star.     In  later  life  the 


THE    LENS 


*H 


•■■»■ 


Fig.  269.— Sections  through  the  Lens  (4)  of  Human  Embryo  of  Thirty 
Thirty-one  Days  and  (B)  of  Pig  Embryo  of  36  Mu.—(Rabl.) 


45: 


THE    LENS 


stars  become  more  complicated,   being  either  six-rayed  or  more 
usually  nine-rayed  in  the  adult  condition '(Fig.  270). 

As  early  as  the  second  month  of  development  the  lens  vesicle 
becomes  completely  invested  by  the  mesodermal  tissue  in  which 
blood-vessels  are  developed  in  considerable  numbers,  whence  the 


Fig.  270.- 


-Posterior  (Inner)  Surface  of  the  Lens  from  an  Adult  showing  the 
Sutural  Lines. — (Rabl.) 


investment  is  termed  the  tunica  vasculosa  lends  (Fig.  278,  tv).  The 
arteries  of  the  tunic  are  in  connection  principally  with  the  hyaloid 
artery  of  the  vitreous  humor  (Fig.  276),  and  consist  of  numerous 
fine  branches  which  envelop  the  lens  and  terminate  in  loops  almost 
at  the  center  of  its  outer  surface.     This  tunic  undergoes  degenera- 


the  optic  cup  453 

tion  after  the  seventh  month  of  development,  by  which  time  the 
lens  has  completed  its  period  of  most  active  growth,  and,  as  a  rule, 
completely  disappears  before  birth.  Occasionally,  however,  it  may 
persist  to  a  greater  or  less  extent,  the  persistence  of  the  portion  cover- 
ing the  outer  surface  of  the  lens,  known  as  the  membrana  papil- 
laris, causing  the  malformation  known  as  congenital  atresia  of  the 
pupil. 

In  addition  to  the  vascular  tunic,  the  lens  is  surrounded  by  a 
non-cellular  membrane  termed  the  capsule.  The  origin  of  'this 
structure  is  still  in  doubt,  some  observers  maintaining  that  it  is  a 
product  of  the  investing  mesoderm,  while  others  hold  it  to  be  a  prod- 
uct of  the  lens  epithelium. 

It  is  interesting  from  the  standpoint  of  developmental  mechanics  to 
note  that  W.  H.  Lewis  and  Spemann  have  shown  that  in  the  Am- 
phibia contact  of  the  optic  vesicle  with  the  ectoderm  is  necessary  for  the 
formation  of  the  lens,  and,  furthermore,  if  the  vesicle  be  transplanted  to 
other  regions  of  the  body  of  a  larva,  a  lens  will  be  developed  from  the 
ectoderm  with  which  it  is  then  in  contact,  even  in  the  abdominal  region, 

The  Development  of  the  Optic  Cup.- — When  the  invagination  of 
the  outer  wall  of  the  optic  bulb  is  completed,  the  margins  of  the 
resulting  cup  are  opposite  the  sides  of  the  lens  vesicle  (Fig.  268), 
but  with  the  enlargement  of  the  lens  and  cup  the  margins  of  the 
latter  gradually  come  to  lie  in  front  of— that  is  to  say,  upon  the  outer 
surface  of — the  lens,  forming  the  boundary  of  the  opening  known 
as  the  pupil.  The  lens,  consequently,  is  brought  to  lie  within  the 
mouth  of  the  optic  cup,  and  that  portion  of  the  latter  which  covers 
the  lens  takes  part  in  the  formation  of  the  iris  and  the  adjacent 
ciliary  body,  while  its  posterior  portion  gives  rise  to  the  retina. 

The  chorioidal  fissure  normally  disappears  during  the  sixth  or 
seventh  week  of  development  by  a  fusion  of  its  lips,  and  not  until 
this  is  accomplished  does  the  term  cup  truly  describe  the  form 
assumed  by  the  optic  bulb  after  the  invagination  of  its  outer  wall. 
In  certain  cases  the  lips  of  the  fissure  fail  to  unite  perfectly,  producing 
the  defect  of  the  eye  known  as  coloboma;  this  may  vary  in  its  extent, 
sometimes  affecting  both  the  iris  and  the  retina  and  forming  what 


454  THE    miS  AND    CILIARY    BODY 

is  termed  coloboma  iridis,  and  at  others  being  confined  to  the  reti- 
nal portion  of  the  cup,  in  which  case  it  is  termed  coloboma 
chorioidae. 

Up  to  a  certain  stage  the  differentiation  of  the  two  layers  which 
form  the  optic  cup  proceeds  along  similar  lines,  in  both  the  ciliary 
and  retinal  regions.  The  layer  which  represents  the  original  inter- 
nal portion  of  the  bulb  does  not  thicken  as  the  cup  increases  in  size, 
and  becomes  also  the  seat  of  a  deposition  of  dark  pigment,  whence 
it  may  be  termed  the  pigment  layer  of  the  cup;  while  the  other  layer — ■ 
that  formed  by  the  invagination  of  the  outer  portion  of  the  bulb,  and 
which  may  be  termed  the  retinal  layer — remains  much  thicker  (Fig. 
268)  and  in  its  proximal  portions  even  increases  in  thickness. 
Later,  however,  the  development  of  the  ciliary  and  retinal  portions 
of  the  retinal  layers  differs,  and  it  will  be  convenient  to  consider 
first  the  history  of  the  ciliary  portion. 

The  Development  of  the  Iris  and  Ciliary  Body. — The  first  change 
noticeable  in  the  ciliary  portion  of  the  retinal  layer  is  its  thinning  out, 
a  process  which  continues  until  the  layer  consists,  like  the  pigment 
layer,  of  but  a  single  layer  of  cells  (Fig.  271),  the  transition  of  which 
to  the  thicker  retinal  portion  of  the  layer  is  somewhat  abrupt  and 
corresponds  to  what  is  termed  the  ora  serrata  in  adult  anatomy. 
In  embryos  of  10.2  cm.  the  retinal  layer  throughout  its  entire  extent 
is  readily  distinguishable  from  the  pigment  layer  by  the  absence  in 
it  of  all  pigmentation,  but  in  older  forms  this  distinction  gradually 
diminishes  in  the  iris  region,  the  retinal  layer  there  acquiring  pig- 
ment and  forming  the  uvea. 

When  the  anterior  chamber  of  the  eye  is  formed  by  the  splitting 
of  the  mesoderm  which  has  grown  in  between  the  superficial  ecto- 
derm and  the  outer  surface  of  the  lens,  the  peripheral  portions  of  its 
posterior  (inner)  wall  are  in  relation  with  the  ciliary  portion  of  the 
optic  cup  and  give  rise  to  the  stroma  of  the  ciliary  body  and  of  the 
iris  (Fig.  271),  this  latter  being  continuous  with  the  tunica  vasculosa 
lentis  so  long  as  that  structure  persists  (Fig.  278).  In  embryos 
of  about  14.5  cm.  the  ciliary  portion  of  the  cup  becomes  thrown  into 
radiating  folds  (Fig.  271),  as  if  by  a  too  rapid  growth,  and  into  the 


THE    IRIS   AND    CILIARY   BODY  455 

folds  lamellae  of  mesoderm  project  from  the  stroma.  These  folds 
occur  not  only  throughout  the  region  of  the  ciliary  body,  but  also 
extend  into  the  iris  region,  where,  however,  they  are  but  temporary 
structures,  disappearing  entirely  by  the  end  of  the  fifth  month.  The 
folds  in  the  region  of  the  corpus  ciliare  persist  and  produce  the 
ciliary  processes  of  the  adult  eye. 

Embedded  in  the  substance  of  the  iris  stroma  in  the  adult  are 
non-striped  muscle-fibers,  which  constitute  the  sphincter  and  dila- 

I-Str 


AE 


CC 
Fig.  271. — Radial  Section  through  the  Iris  of  an  Embryo  of  19  cm. 
AE,  Pigment  layer;  CC,  ciliary  folds;  IE,  retinal  layer;  I.Str,  iris  stroma;  Pm,  pupillary 
membrane;  Rs,  marginal  sinus;  Sph,  sphincter  iridis. — (Szili.) 

tator  iridis.  It  has  long  been  supposed  that  these  fibers  were  dif- 
ferentiated from  the  stroma  of  the  iris,  but  recent  observations  have 
shown  that  they  arise  from  the  cells  of  the  pigment  layer  of  the  optic 
cup,  the  sphincter  appearing  near  the  pupillary  border  (Fig.  271, 
Sph)  while  the  dilatator  is  more  peripheral. 

The  Development  of  the  Retina. — Throughout  the  retinal  region 
of  the  cup  the  pigment  layer,  undergoing  the  same  changes  as  in 


456 


THE    RETINA 


the  ciliary  region,  forms  the  pigment  layer  of  the  retina  (Fig.  272,  p). 
The  retinal  layer  increases  in  thickness  and  early  becomes  differen- 
tiated into  two  strata  (Fig.  268),  a  thicker  one  lying  next  the  pigment 
layer  and  containing  numerous  nuclei,  and  a  thinner  one  containing 
no  nuclei.  The  thinner  layer,  from  its  position  and  structure, 
suggests  an  homology  with  the  marginal  velum  of  the  central  nervous 
system,  and  probably  becomes  converted  into  the  nerve-fiber  layer 


'0%&BoS' 


00  0°  o  o 


o 


o 


Fig.  272. — Portion  of  a  Transverse  Section  of  the  Retina  of  a  New-born 

Rabbit. 
ch,  Chorioid  coat;  g,  ganglion-cell  layer;  r,  outer  layer  of  nuclei;  p,  pigment  layer. — 

(Falchi.) 

of  the  adult  retina,  the  axis-cylinder  processes  of  the  ganglion  cells 
passing  into  it  on  their  way  to  the  optic  nerve.  The  thicker  layer 
similarly  suggests  a  comparison  with  the  mantle  layer  of  the  cord 
and  brain,  and  in  embryos  of  38  mm.  it  becomes  differentiated  into 
two  secondary  layers  (Fig.  272),  that  nearest  the  pigment  layer 
if)  consisting  of  smaller  and  more  deeply  staining  nuclei,  probably 
representing  the  rod  and  cone  and  bipolar  cells  of  the  adult  retina, 


THE    RETINA 


457 


while  the  inner  layer,  that  nearest  the  marginal  velum,  has  larger 
nuclei  and  is  presumably  composed  of  the  ganglion  cells. 

Little  is  as  yet  known  concerning  the  further  differentiation  of 
the  nervous  elements  of  the  human  retina,  but  the  history  of  some 
of  them  has  been  traced  in  the  cat,  in  which,  as  in  other  mammals, 
the  histogenetic  processes  take  place  at  a  relatively  later  period  than 
in  man.     Of  the  histogenesis  of  the  inner  layer  the  information  is 


Fig.  273. — Diagram  showing  the  Development  of  the  Retinal  Elements. 

a,  Cone  cell  in  the  unipolar,  and  b,  in  the  bipolar  stage;  c,  rod  cells  in  the  unipolar, 
and  d,  in  the  bipolar  stage;  e,  bipolar  cells; /and  i,  amacrine  cells;  g,  horizontal  cells; 
h,  ganglion  cells;  k,  Muller's  fiber;  I,  external  limiting  membrane. — (Kallius,  after 
Cajal.) 

rather  scant,  but  it  may  be  stated  that  the  ganglion  cells  are  the 
earliest  of  all  the  elements  of  the  retina  to  become  recognizable. 
The  rod  and  cone  cells,  when  first  distinguishable,  are  unipolar  cells 
(Fig.  273,  a  and  c),  their  single  processes  extending  outward  from  the 
cell-bodies  to  the  external  limiting  membrane  which  bounds  the 
outer  surface  of  the  retinal  layer.  Even  at  an  early  stage  the  cone 
cells  (a)  are  distinguishable  from  the  rod  cells  (c)  by  their  more 


458  THE    OPTIC   NERVE 

decided  reaction  to  silver  salts,  and  at  first  both  kinds  of  cells  are 
scattered  throughout  the  thickness  of  the  layer  from  which  they  arise. 
Later,  a  fine  process  grows  out  from  the  inner  end  of  each  cell,  which 
thus  assumes  a  bipolar  form  (Fig.  2  73 ,  b  and  d) ,  and,  later  still,  the  cells 
gradually  migrate  toward  the  external  limiting  membrane,  beneath 
which  they  form  a  definite  layer  in  the  adult.  In  the  meantime 
there  appears  opposite  the  outer  end  of  each  cell  a  rounded  eminence 
projecting  from  the  outer  surface  of  the  external  limiting  membrane 
into  the  pigment  layer.  The  eminences  over  the  cone  cells  are  larger 
than  those  over  the  rod  cells,  and  later,  as  both  increase  in  length, 
they  become  recognizable  by  their  shape  as  the  rods  and  cones. 

The  bipolar  cells  are  not  easily  distinguishable  in  the  early  stages 
of  their  differentiation  from  the  other  cells  with  which  thy  are  min- 
gled, but  it  is  believed  that  they  are  represented  by  cells  which  are 
bipolar  when  the  rod  and  cone  cells  are  still  in  a  unipolar  condition 
(Fig.  273,  e).  If  this  identification  be  correct,  then  it  is  noteworthy 
that  at  first  their  outer  processes  extend  as  far  as  the  external  limiting 
membrane  and  must  later  shorten  or  fail  to  elongate  until  their 
outer  ends  lie  in  what  is  termed  the  outer  granular  layer  of  the  retina, 
where  they  stand  in  relation  to  the  inner  ends  of  the  rod  and  cone 
cell  processes.  Of  the  development  of  the  amacrine  (/",  i)  and 
horizontal  cells  (g)  of  the  retina  little  is  known.  From  their  position 
in  new-born  kittens  it  seems  probable  that  the  former  are  derived 
from  cells  of  the  same  layer  as  the  ganglion  cells,  while  the  horizon- 
tal cells  may  belong  to  the  outer  layer. 

In  addition  to  the  various  nerve-elements  mentioned  above,  the 
retina  also  contains  neuroglial  elements  known  as  Muller's  fibers 
(Fig.  273,  k),  which  traverse  the  entire  thickness  of  the  retina.  The 
development  of  these  cells  has  not  yet  been  thoroughly  traced,  but 
they  resemble  closely  the  ependymal  cells  observable  in  early  stages 
of  the  spinal  cord. 

The  Development  of  the  Optic  Nerve. — The  observations  on  the 
development  of  the  retina  have  shown  very  clearly  that  the  great 
majority  of  the  fibers  of  the  optic  nerve  are  axis-cylinders  of  the  gang- 
lion cells  of  the  retina  and  grow  from  these  cells  along  the  optic 


THE    OPTIC   NERVE 


459 


stalk  toward  the  brain.  Their  embryonic  history  has  been  traced 
most  thoroughly  in  rat  embryos  (Robinson),  and  what  follows  is 
based  upon  what  has  been  observed  in  that  animal. 

The  optic  stalk,  being  an  outgrowth  from  the  brain,  is  at  first 
a  hollow  structure,  its  cavity  communicating  with  that  of  the  third 
ventricle  at  one  end  and  with  that  of 
the  optic  bulb  at  the  other.  When  the 
chorioid  fissure  is  developed,  it  extends, 
as  has  already  been  described,  for  some 
distance  along  the  posterior  surface  of 
the  stalk  and  has  lying  in  it  a  portion  of 
the  hyaloid  artery.  Later,  when  the  lips 
of  the  fissure  fuse,  the  artery  becomes 
enclosed  within  the  stalk  to  form  the  ar- 
teria  centralis  retina  of  the  adult  (Fig. 
276).  By  the  formation  of  the  fissure 
the  original  cavity  of  the  distal  portion 
of  the  stalk  becomes  obliterated,  and  at 
the  same  time  the  ventral  and  posterior 
walls  of  the  stalk  are  brought  into  con- 
tinuity with  the  retinal  layer  of  the  op- 
tic cup,  and  so  opportunity  is  given  for  the  passage  of  the 
axis-cylinders  of  the  ganglion  cells  along  those  walls  (Fig.  274). 
At  an  early  stage  a  section  of  the  proximal  portion  of  the  optic 
stalk  (Fig.  275,  A)  shows  the  central  cavity  surrounded  by  a  num- 
ber of  nuclei  representing  the  mantle  layer,  and  surrounding 
these  a  non-nucleated  layer,  resembling  the  marginal  velum  and 
continuous  distally  with  the  similar  layer  of  the  retina.  When  the 
ganglion  cells  of  the  latter  begin  to  send  out  their  axis-cylinder 
processes,  these  pass  into  the  retinal  marginal  velum  and  converge 
in  this  layer  toward  the  bottom  of  the  chorioidal  fissure,  so  reaching 
the  ventral  wall  of  the  optic  stalk,  in  the  velum  of  which  they  may 
be  distinguished  in  rat  embryos  of  4  mm.,  and  still  more  clearly  in 
those  of  9  mm.  (Fig.  275,  A).  Later,  as  the  fibers  become  more 
numerous,  they  gradually  invade  the  lateral  and  finally  the  dorsal 


Fig.  2  74. — Diagrammatic 
Longitudinal  Section  of  the 
Optic  Cup  and  Stalk  passing 
through  the  chorioid  fis- 
SURE. 

Ah,  Hyaloid  artery;  L,  lens; 
On,  fibers  of  the  optic  nerve;  Os, 
optic  stalk;  PI,  pigment  layer, 
and  R,  retinal  layer  of  the  retina. 


460  THE    OPTIC   NERVE 

walls  of  the  stalk,  and,  at  the  same  time  the  mantle  cells  of  the  stalk 
become  more  scattered  and  assume  the  form  of  connective-tissue 
(neuroglia)  cells,  while  the  original  cavity  of  the  stalk  is  gradually 
obliterated  (Fig.  275,  B).  Finally,  the  stalk  becomes  a  solid  mass 
of  nerve-fibers,  among  which  the  altered  mantle  cells  are  scattered. 

From  what  has  been  stated  above  it  will  be  seen  that  the  sensory 
cells  of  the  eye  belong  to  a  somewhat  different  category  from  those  of  the 
other  sense-organs.  Embryologically  they  are  a  specialized  portion  of  the 
mantle  layer  of  the  medullary  canal,  whereas  in  the  other  organs  they  are 
peripheral  structures  either  representing  or  being  associated  with  repre- 
sentatives of  posterior  root  ganglion  cells.  Viewed  from  this  standpoint, 
and  taking  into  consideration  the  fact  that  the  sensory  portion  of  the 
retina  is  formed  from  the  invaginated  part  of  the  optic  bulb,  some  light 


":  .      ■■■'V    ,".-■■' 


Fig.  275. — Transverse  Sections  through  the  Proximal  Part  of  the  Optic  Stalk 
of  Rat  Embryos  of  (A)  9  mm.  and  (5)  11  mm. — (Robinson.) 

is  thrown  upon  the  inverted  arrangement  of  the  retinal  elements,  the  rods 
and  cones  being  directed  away  from  the  source  of  light.  The  normal 
relations  of  the  mantle  layer  and  marginal  velum  are  retained  in  the  retina, 
and  the  latter  serving  as  a  conducting  layer  for  the  axis-cylinders  of  the 
mantle  layer  (ganglion)  cells,  the  layer  of  nerve-fibers  becomes  interposed 
between  the  source  of  light  and  the  sensory  cells.  Furthermore,  it 
may  be  pointed  out  that  if  the  differentiation  of  the  retina  be  im- 
agined to  take  place  before  the  closure  of  the  medullary  canal — a 
condition  which  is  indicated  in  some  of  the  lower  vertebrates — there 
would  be  then  no  inversion  of  the  elements,  this  peculiarity  being  due  to 
the  conversion  of  the  medullary  plate  into  a  tube,  and  more  especially  to 
the  fact  that  the  retina  develops  from  the  outer  wall  of  the  optic  cup.     In 


THE    VITREOUS   HUMOR 


461 


certain  reptiles  in  which  an  eye  is  developed  in  connection  with  the  epiphy- 
sial outgrowths  of  the  diencephalon,  the  retinal  portion  of  this  pineal  eye 
is  formed  from  the  inner  layer  of  the  bulb,  and  in  this  case  there  is  no 
inversion  of  the  elements. 

A  justification  of  the  exclusion  of  the  optic  nerve  from  the  category 
which  includes  the  other  cranial  nerves  has  now  been  presented.  For  if 
the  retina  be  regarded  as  a  portion  of  the  central  nervous  system,  it  is  clear 
that  the  nerve  is  not  a  nerve  at  all  in  the  strict  sense  of  that  word,  but  is  a 
tract,  confined  throughout  its  entire  extent  within  the  central  nervous 
system  and  comparable  to  such  groups  of  fibers  as  the  direct  cerebellar 
or  fillet  tracts  of  that  system. 

The  Development  of  the  Vitreous  Humor. — It  has  already  been 
pointed  out  (p.  448)  that  a  blood-vessel,  the  hyaloid  artery,  accom- 
panied by  some  mesodermal  tissue  makes  its  way  into  the  cavity 


Fig.  276. — Reconstruction  of  a  Portion  of  the  Eye  of  an  Embryo  of  13.8  mm. 
ah,  Hyaloid  artery;  ch,  chorioid  coat;  /,  lens;  r,  retina. — (His.) 

of  the  optic  cup  through  the  chorioid  fissure.  On  the  closure  of  the 
fissure  the  artery  becomes  enclosed  within  the  optic  stalk  and  appears 
to  penetrate  the  retina,  upon  the  surface  of  which  its  branches 
ramify.  In  the  embryo  the  artery  does  not,  however,  terminate 
in  these  branches  as  it  does  in  the  adult,  but  is  continued  on  through 
the  cavity  of  the  optic  cup  (Fig.  276)  to  reach  the  lens,  around  which 
it  sends  branches  to  form  the  tunica  vasculosa  lentis. 

According  to  some  authors,  the  formation  of  the  vitreous  humor 
is  closely  associated  with  the  development  of  this  artery,  the  humor 
being  merely  a  transudate  from  it,  while  others  have  maintained 
that  it  is  a  derivative  of  the  mesoderm  which  accompanies  the  vessel, 
and  is  therefore  to  be  regarded  as  a  peculiar  gelatinous  form  of 


462 


THE    VITREOUS   HUMOR 


connective  tissue.  More  recently,  however,  renewed  observations 
by  several  authors  have  resulted  in  the  deposition  of  the  mesoderm 
from  the  chief  role  in  the  formation  of  the  vitreous  and  the  substitu- 
tion in  it  of  the  retina.  At  an  early  stage  of  development  delicate 
protoplasmic  processes  may  be  seen  projecting  from  the  surface  of 
the  retinal  layer  into  the  cavity  of  the  optic  cup,  these  processes 
probably  arising  from  those  cells  which  will  later  form  the  Miiller's 


Fig.  277. — Transverse  Section  through  the  Ciliary  Region  of  a  Chick  Embryo 

of  Sixteen  Days. 
ac,  Anterior  chamber  of  the  eye;  cj,  conjunctiva;  co,  cornea;  i,  iris;  I,  lens;  mc,  ciliary 
muscle;  rl,  retinal  layer  of  optic  cup;  sf,  spaces  of  Fontana;  si,  suspensory  ligament  of  the 
lens;  v,  vitreous  humor. — (Angelucci.) 

(neuroglia)  fibers  of  the  retina.  As  development  proceeds  they  in- 
crease in  length,  forming  a  dense  and  very  fine  fibrillar  reticulum 
traversing  the  space  between  the  lens  and  the  retina  and  constituting 
the  primary  vitreous  humor.  The  formation  of  the  fibers  is  espe- 
cially active  in  the  ciliary  portion  of  the  retina  and  it  is  probable  that 
it  is  from  some  of  the  fibers  developing  in  this  region  that  the  sus- 
pensory ligament  of  the  lens  (zonula  Zinnii)  (Fig.  277,  si)  is  formed 


THE    CORNEA  463 

spaces  which  occur  between  the  fibers  of  the  ligament  enlarging  to 
produce  a  cavity  traversed  by  scattered  fibers  and  known  as  the 
canal  of  Petit. 

A  participation  of  similar  protoplasmic  prolongations  from  the 
cells  of  the  lens  in  the  formation  of  the  vitreous  humor  has  been 
maintained  (von  Lenhossek)  and  as  strenuously  denied.  But  it  is 
generally  admitted  that  at  the  time  when  the  hyaloid  artery  pene- 
trates the  vitreous  to  form  the  tunica  vasculosa  lentis  it  carries  with 
it  certain  mesodermal  elements,  whose  fate  is  at  present  uncertain. 
It  has  been  held  that  they  take  part  in  the  formation  of  the  definitive 
vitreous,  which,  according  to  this  view,  is  of  mixed  origin,  being 
partly  ectodermal  and  partly  mesodermal  (Van  P6e),  and,  on  the 
contrary,  it  has  been  maintained  that  they  eventually  undergo 
complete  degeneration,  the  vitreous  being  of  purely  ectodermal 
origin  (von  Kolliker). 

The  degeneration  of  the  mesodermal  elements  which  the  latter 
view  supposes  is  associated  with  the  degeneration  of  the  hyaloid 
artery.  This  begins  in  human  embryos  in  the  third  month  and  is 
completed  during  the  ninth  month,  the  only  trace  after  birth  of  the 
existence  of  the  vessel  being  a  more  fluid  consistency  of  the  axis  of 
the  vitreous  humor,  this  more  fluid  portion  representing  the  space 
originally  occupied  by  the  artery  and  forming  what  is  termed  the 
hyaloid  canal  {canal  ofCloquet). 

The  Development  of  the  Outer  Coat  of  the  Eye,  of  the  Cornea,  and 
of  the  Anterior  Chamber. — Soon  after  the  formation  of  the  optic  bulb 
a  condensation  of  the  mesoderm  cells  around  it  occurs,  forming  a 
capsule.  Over  the  medial  portions  of  the  optic  cup  the  further 
differentiation  of  this  capsule  is  comparatively  simple,  resulting  in 
the  formation  of  two  layers,  an  inner  vascular  and  an  outer  denser 
and  fibrous,  the  former  becoming  the  chorioid  coat  of  the  adult  eye 
and  the  latter  the  sclera. 

More  laterally,  however,  the  processes  are  more  complicated. 
After  the  lens  has  separated  from  the  surface  ectoderm  a  thin  layer 
of  mesoderm  grows  in  between  the  two  structures  and  later  gives 
place  to  a  layer  of  homogeneous  substance  in  which  a  few  cells, 


464 


THE   ANTERIOR    CHAMBER    OF   THE  EYE 


more  numerous  laterally  than  at  the  center,  are  embedded.  Still 
later  cells  from  the  adjacent  mesenchyme  grow  into  the  layer,  which 
increases  considerably  in  thickness,  and  blood-vessels  also  grow  into 
that  portion  of  it  which  is  in  contact  with  the  outer  surface  of  the 
lens.  At  this  stage  the  interval  between  the  surface  ectoderm  and 
the  lens  is  occupied  by  a  solid  mass  of  mesodermal  tissue  (Fig.  278, 
co  and  tv),  but  as  development  proceeds,  small  spaces  (ac)  filled 

etc 


ec^ 


mc 


Fig.  278. — Transverse  Section  through  the  Ciliary  Region  of  a  Pig  Embryo  or 

23  MM. 
ac,  Anterior  chamber  of  the  eye;  co,  cornea;  ec,  ectoderm;  I,  lens;  mc,  ciliary  muscle; 
p,  pigment  layer  of  the  optic  cup;  r,  retinal  layer;  tv,  tunica  vasculosa  lentis. — (Angelucci.) 

with  fluid  begin  to  appear  toward  the  inner  portion  of  the  mass,  and 
these,  increasing  in  number  and  size,  eventually  fuse  together  to 
form  a  single  cavity  which  divides  the  mass  into  an  inner  and  an 
outer  portion.  The  cavity  is  the  anterior  chamber  of  the  eye,  and  it 
has  served  to  separate  the  cornea  (co)  from  the  tunica  vasculosa 
lentis  (tv) ,  and,  extending  laterally  in  all  directions,  it  also  separates 
from  the  cornea  the  mesenchyme  which  rests  upon  the  marginal 
portion  of  the  optic  cup  and  constitutes  the  stroma  of  the  iris.  Cells 
arrange  themselves  on  the  corneal  surface  of  the  cavity  to  form  a 


THE    EYELIDS  465 

continuous  endothelial  layer,  and  the  mesenchyme  which  forms  the 
peripheral  boundary  of  the  cavity  assumes  a  fibrous  character  and 
forms  the  ligamentum  pectinatum  iridis,  among  the  fibers  of  which 
cavities,  known  as  the  spaces  of  Fontana  (Fig.  277,  sf),  appear. 
Beyond  the  margins  of  the  cavity  the  corneal  tissue  is  directly  con- 
tinuous with  the  sclerotic,  beneath  the  margin  of  which  is  a  distinctly 
thickened  portion  of  mesenchyme  resting  upon  the  ciliary  processes 
and  forming  the  stroma  of  the  ciliary  body,  as  well  as  giving  rise  to 
the  muscle  tissue  which  constitutes  the  ciliary  muscle  (Figs.  277  and 
278,  mc). 

The  ectoderm  which  covers  the  outer  surface  of  the  eye  does  not 
proceed  beyond  the  stage  when  it  consists  of  several  layers  of  cells, 
and  never  develops  a  stratum  corneum.  In  the  corneal  region  it 
rests  directly  upon  the  corneal  tissue,  which  is  thickened  slightly 
upon  its  outer  surface  to  form  the  anterior  elastic  lamina;  more  per- 
ipherally, however,  a  quantity  of  loose  mesodermal  tissue  lies 
between  the  ectoderm  and  the  outer  surface  of  the  sclerotic,  and, 
together  with  the  ectoderm,  forms  the  conjunctiva  (Fig.  277,  cj). 

The  Development  of  the  Accessory  Apparatus  of  the  Eye. — The 
eyelids  make  their  appearance  at  an  early  stage  as  two  folds  of  skin, 
one  a  short  distance  above  and  the  other  below  the  cornea.  The 
center  of  the  folds  is  at  first  occupied  by  indifferent  mesodermal 
tissue,  which  later  becomes  modified  to  form  the  connective  tissue 
of  the  lids  and  the  tarsal  cartilage,  the  muscle  tissue  probably 
secondarily  growing  into  the  lids  as  a  result  of  the  spreading  of  the 
platysma  over  the  face,  the  orbicularis  oculi  apparently  being  a 
derivative  of  that  sheet  of  muscle  tissue. 

At  about  the  beginning  of  the  third  month  the  lids  have  become 
sufficiently  large  to  meet  one  another,  whereupon  the  thickened 
epithelium  which  has  formed  upon  their  edges  unites  and  the  lids 
fuse  together,  in  which  condition  they  remain  until  shortly  before 
birth.  During  the  stage  of  fusion  the  eyelashes  (Fig.  279,  h)  develop 
at  the  edges  of  the  lids,  having  the  same  developmental  history  as 
ordinary  hairs,  and  from  the  fused  epithelium  of  each  lid  there  grow 
upward  or  downward,  as  the  case  may  be,  into  the  mesodermic 
3° 


466 


THE   EYELIDS 


tissue,  solid  rods  of  ectoderm,  certain  of  which  early  give  off  numer- 
ous short  lateral  processes  and  become  recognizable  as  the  tarsal 
{Meibomian)  glands  (m),  while  others  retain  the  simple  cylindrical 
form  and  represent  the  glands  of  Moll.  When  the  eyelids  separate, 
these  solid  ingrowths  become  hollow  by  a  breaking  down  of  their 


Fig.  279. — Section  through  the  Margins  of  the  Fused  Eyelids  in  an  Embryo^ 

of  Six  Months.  i  "1 


h,  Eyelash;  //,  lower  lid;  m,  tarsal  gland;  mu,  muscle  bundle;  ul,  upper  lid.- 

Seidl.) 


-{Schweigger 


central  cells,  just  as  in  the  sebaceous  and  sudoriparous  glands  of 
the  skin,  the  tarsal  glands  being  really  modifications  of  the  former 
glands,  while  the  glands  of  Moll  are  probably  to  be  regarded  as 
specialized  sudoriparous  glands. 

A  third  fold  of  skin,  in  addition  to^the  two  which  produce  the 
eyelids,  is  also  developed  in  connection  with  the  eye,  forming  the 
plica  semilunaris.  This  is  a  rudimentary  third  eyelid,  representing 
the  nictitating  membrane  which  is  fairly  well  developed  in  many 
of  the  lower  mammals  and  especially  well  in  birds. 


THE    LACHRYMAL    GLAND  467 

The  lachrymal  gland  is  developed  at  about  the  third  month  as  a 
number  of  branching  outgrowths  of  the  ectoderm  into  the  adjacent 
mesoderm  along  the  outer  part  of  the  line  where  the  epithelium  of 
the  conjunctiva  becomes  continuous  with  that  covering  the  inner 
surface  of  the  upper  eyelid.  As  in  the  other  epidermal  glands,  the 
outgrowths  and  their  branches  are  at  first  solid,  later  becoming  hol- 
low by  the  degeneration  of  their  axial  cells. 

The  naso-lachrymal  duct  is  developed  in  connection  with  the 
groove  which,  at  an  early  stage  in  the  development  (Fig.  62),  extends 


Fig.  280. — Diagram  showing  the  Insertions  of  the  Lachrymal  Ducts  in- 

EMBRYOS   OF  40  MM.  AND    170  MM..   THE  CaRUNCULA   LaCRIMALIS    BEING   FORMED   IN 

the  Latter. 

The  eyelids  are  really  fused  at  these  stages  but  have  been  represented  as  separate 
•  for  the  sake  of  clearness. — (Ask.) 

from  the  inner  corner  of  the  eye  to  the  olfactory  pit  and  is  bounded 
posteriorly  by  the  maxillary  process  of  the  first  visceral  arch.  The 
epithelium  lying  in  the  floor  of  this  groove  thickens  toward  the  begin- 
ing  of  the  sixth  week  to  form  a  solid  cord,  which  sinks  into  the  sub- 
jacent mesoderm.  From  its  upper  end  two  outgrowths  arise  which 
become  connected  with  the  ectoderm  of  the  edges  of  the  upper  and 
lower  lids,  respectively,  and  represent  the  lachrymal  ducts,  and, 
finally,  the  solid  cord  and  its  outgrowths  acquire  a  lumen  and  a 
connection  with  the  mucous  membrane  of  the  inferior  meatus  of  the 
nasal  cavity. 

The  inferior  duct  connects  with  the  border  of  the  eyelid  some 
distance  lateral  to  the  inner  angle  of  the  eye,  and  between  its  open- 
ing and  the  angle  a  number  of  tarsal  glands  develop.  The  superior 
duct,  on  the  other  hand,  opens  at  first  close  to  the  inner  angle  and 


468  LITERATURE 

later  moves  laterally  until  its  opening  is  opposite  that  of  the  inferior 
duct.  During  this  change  the  portion  of  the  lower  lid  between  the 
opening  of  the  inferior  duct  and  the  angle  is  drawn  somewhat  up- 
ward, and,  with  its  glands,  forms  a  small  reddish  nodule,  resting 
upon  the  plica  semilunaris  and  known  as  the  caruncula  lacrimalis 
(Fig.  280). 

LITERATURE. 

G.  Alexander:  "Ueber  Entwicklung  und  Bau  des  Pars  inferior  Labyrinthi  der 
hdheren  Saugethiere,"  Denkschr.  kais.  wissench.  Acad.  Wien,  Math.-Naturw. 
Classe,  lxx,  1901. 

A.  Angeltjcci:  "Ueber  Entwickelung  und  Bau  des  vorderen  Uvealtractus  der  Verte- 

braten,"  Archiv  fur  mikrosk.  Anat.,  xix,  1881. 
F.  Ask:  "  Ueber  die  Entwickelung  der  Caruncula  lacrimalis  beim  Menschen,  nebst 
Bemerkungen  iiber  die  Entwickelung  der  Tranenrohrchen  und  der  Meibom'schen 
Driisen,"  Anatom.  Anzeiger,  xxx,  1907. 

F.  Ask:  "Ueber  die  Entwicklung  der  Lidrander,  der  Tranenkarunkel  und  der  Nick- 

haut  beim  Menschen,  nebst  Bemerkungen  zur  Entwicklung  der  Tranenabfuhr- 
ungswege,"  Anat.  Hefte,  xxxvi,  1908. 

B.  Baginsky:  "Zur  Entwickelung  der  Gehorschnecke,"  Archiv  fur  mikrosk.  Anat., 

xxviii,  1886. 
I.  Broman:  "Die  Entwickelungsgeschichte  der  Gehorknochelchen  beim  Menschen," 

Anat.  Hefte,  xi,  189S. 
S.  Ramon  y  Cajal:  "Nouvelles  contributions  a  l'etude  histologique  de  la  retine," 

Journ.  de  I' Anat.  et  de  la  Physiol.,  xxxii,  1896. 

G.  Cirincione:  "Ueber  den  gegenwartigen  Stand  der  Frage  hinsichtlich  der  Genese 

des  Glaskorpers,"  Arch,  fur  Augenheilk.,  L,  1904. 
A.  Contino:  "Ueber  Bau  and  Entwicklung  des  Lidrandes  beim  Menschen,"  Arch, 
fur  Ophthalmol.,  lxvi,  1908. 

A.  Contino:  "Ueber  die  Entwicklung  der  Karunkel  und  der  plica  semilunaris  beim 

Menschen,"  Arch,  fur  Ophthalmol,  lxxi,  1909. 
J.  Disse:  "Die  erste  Entwickelung  der  Riechnerven,"  Anat.  Hefte,  ix,  1897. 

B.  Fleischer:  "Die  Entwickelung  der  Tranenrohrchen  bei  den  Saugetiere,"  Archiv 

fur  Ophthalmol.,  lxii,  1906. 
H.  Fuchs  :  "  Bemerkungen  iiber  die  Herkunft  und  Entwickelung  der  Gehorknochelchen 

bei    Kaninchen-Embryonen    (nebst   Bemerkungen   iiber   die   Entwickelung   des 

Knorpelskeletes  der  beiden  ersten  Visceralbogen),"  Archiv.  fur  Anat  und  Phys., 

Anat.  Abth.,  Supplement,  1905. 
J.  Graberg:  "Beitrage  zur  Genese  des  Geschmacksorgans  der  Menschen,"  Morphol. 

Arbeiten,  vn,  1898. 
J.  A.  Hammar:  "Zur  allgemeinen  Morphologie  der  Schlundspalten  des  Menschen. 

Zur  Entwickelungsgeschichte  des  Mittelohrraumes,   des  ausseren   Gehorganges 

und  des  Paukenfelles  beim  Menschen,"  Anat.  Anzeiger,  xx,  1901. 


LITERATURE 


469 


J.  A.  Hammar:  "  Studien  iiber  die  Entwicklung  des  Vorderdarms  und  einiger  angrenz- 

ender  Organe,"  Arch,  fur  mikrosk.  Anat.,  Lix,  1902. 
C.   Heerfordt:   "Studien   iiber  den   Muse,   dilatator  pupilke   sammt  Angabe  von 

gemeinschaftlicher   Kennzeichen    einiger  Falle   epithelialer   Musculatur,"    Anat. 

Hefte,  xiv. 
J.  Hegetschweiler:  "Die  embryologische  Entwickelung  des  Steigbugels,"  Archiv 

fur  Anat.  und  Physiol.,  Anat.  Abth.,  1898. 

F.  Hochstetter:  "Ueber  die  Bildung  der  primitiven  Choanen  beim  Menschen," 

Verhandl.  Anat.  Gesellsch.,  VI,  1892. 
W.    His,    Jr.:    "Die    Entwickelungsgeschichte    des    Acustico-Facialisgebietes    beim 

Menschen,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,  1897. 
A.  von  Kolliker:  "Die  Entwicklung  und  Bedeutung  des  Glaskorpers,"  Zeitschr. 

fur  wissensch.  Zoolog.,  lxxvi,  1904. 
P.  Lang:  "Zur  Entwicklung  des  Tranenausfiihrsapparates  beim  Menschen,"  Anat. 

Anzeiger,"  xxxvni,  1911. 

G.  Leboucq:  "  Contribution  a.  l'etude  de  l'histogenese  de  la  retine  chez  les  mammiferes," 

Arch.  Anat.,  Microsc,  x,  1909. 
V.  von  Mihalkovicz:  "Nasenhohle  und  Jacobsonsches  Organ.     Eine  morphologische 

Studie."  Anat.  Hefte.  xi,  1898. 
J.   L.  Paitlet:   "Contribution  a  l'etude  de  l'organe  de   Jacobson  chez  l'embryon 

humain,"  Bibliogr.  Anat.,  xvii,  1907. 
P.  van  Pee:  "Recherches  sur  l'origine  du  corps  vitre,"  Archives  de  Biol.,  xix,  1902. 
C.  Rabl:  "Ueber  den  Bau  und  Entwickelung  der  Linse,"  Zeitschrift  fur  wissensch. 

Zoologie,  lxiii  and  lxv,  1898;  lxviii,  1899. 
A.  Robinson:  "On  the  Formation  and  Structure  of  the  Optic  Nerve  and  Its  Relation 

to  the  Optic  Stalk,"  Journal  of  Anat.  and  Physiol.,  xxx,  1896. 
G.  Speciale-Cirincione:  "Ueber  die  Entwicklung  der  Tranendriise  beim  Menschen  " 

Arch.filr  Ophthalmol.,  LXIX,  1908. 
J.  P.  Schaefeer:  "The  Genesis  and  Development  of  the  Nasolacrimal  Passages  in 

Man,"  Amer.  Journ.  Anat.,  xm,  1912. 
G.  L.   Streeter:   "On  the  Development  of  the  Membranous  Labyrinth  and  the 

Acoustic  and  Facial  Nerves  in  the  Human  Embryo,"   Amer.  Journ.  of  Anat. 

vi,  1907. 
N.  van  der  Stricht:  "L'histogenese  des  parties  constituantes  du  neuroepithelium 

acoustique,  des  taches  et  des  cretes  acoustiques  et  de  l'organe  de  Corti  "  Arch. 

de  Biol.,  xxiii,  1908. 
A.  Szili:  "Zur  Anatomie  und  Entwickelungsgeschichte   der   hinteren    Irisschichten 

mit  besonderer  Beriicksichtigung  des  Musculus  sphincter  iridis  des  Menschen  " 

Anat.  Anzeiger,  xx,  1901. 
A.  Szili:  "Ueber  das  Entstehen  eines  fibrillares  Stutzgewebes  im Embryo  und  dessen 

Verhaltnis  zur  Glaskorperfrage,"  Anat.  Hejte,  xxxv,  190S. 
F.  Tuckerman:  "On  the  Development  of  the  Taste  Organs  in  Man,"  Journal  of  Anat. 

and  Physiol.,  xxiv,  1889. 
R.  Versari:    "Ueber  die  Entwicklung  der  Blutgefasse  des  menschlichen  Au^es  " 

Anat.  Anzeiger,  xxxv,  1909. 


CHAPTER  XVII. 
POST-NATAL  DEVELOPMENT. 

In  the  preceding  pages  attention  has  been  directed  principally 
to  the  changes  which  take  place  in  the  various  organs  during  the 
period  before  birth,  for,  with  a  few  exceptions,  notably  that  of  the 
liver,  the  general  form  and  histological  peculiarities  of  the  various 
organs  are  acquired  before  that  epoch.  Development  does  not, 
however,  cease  with  birth,  and  a  few  statements  regarding  the 
changes  which  take  place  in  the  interval  between  birth  and  maturity 
will  not  be  out  of  place  in  a  work  of  this  kind. 

The  conditions  which  obtain  during  embryonic  life  are  so  dif- 
ferent from  those  to  which  the  body  must  later  adapt  itself,  that 
arrangements,  such  as  those  connected  with  the  placental  circula- 
tion, which  are  of  fundamental  importance  during  the  life  in  utero, 
become  of  little  or  no  use,  while  the  relative  importance  of  others  is 
greatly  diminished,  and  these  changes  react  more  or  less  profoundly 
on  all  parts  of  the  body.  Hence,  although  the  post-natal  develop- 
ment consists  chiefly  in  the  growth  of  the  structures  formed  during 
earlier  stages,  yet  the  growth  is  not  equally  rapid  in  all  parts,  and 
indeed  in  some  organs  there  may  even  be  a  relative  decrease  in  size. 
That  this  is  true  can  be  seen  from  the  annexed  figure  (Fig.  281), 
which  represents  the  body  of  a  child  and  that  of  an  adult  man  drawn 
as  of  the  same  height.  The  greater  relative  size  of  the  head  and 
upper  part  of  the  body  in  the  child  is  very  marked,  and  the  central 
point  of  the  height  of  the  child  is  situated  at  about  the  level  of  the 
umbilicus,  while  in  the  man  it  is  at  the  symphysis  pubis. 

That  there  is  a  distinct  change  in  the  geometric  form  of  the  body 
during  growth  is  also  well  shown  by  the  following  consideration. 
(Thoma).  Taking  the  average  height  of  a  new-born  male  as  500 
mm.,  and  that  of  a  man  of  thirty  years  of  age  as  1686  mm.,  the 

470 


POST-NATAL    DEVELOPMENT  _": 

height  of  the  body  will  have  increased  from  birth  to  adolescence 

-  v  _■ 

=  3.37  times.    The  child  will  weigh  3.1  kilos  and  the  man 

5:: 

66.1  kilos,  and  if  the  specific  gravity  of  the  body  with  the  included 

gases  be  taken  in  the  one  case  as  0.90  and  in  the  other  as  0.93,  then 

the  volume  of  the  child's  body  will  be  3.44  liters  and  that  of  the 

man's  71.08  liters,  and  the  increase  in  volume  will  be  — —  =20.66. 


Fig.  281. — Child  ast>  }vL\x  Drawn  as  of  th* 
"  Growth  of  the  Brain, "  Contemporary  Science  Series 
Sons.) 


-     ::      ..-.-;.:  .;-'  S'-.:-'.;:  5r:':    r ' : 


If  the  increase  in  volume  had  taken  place  without  any  alteration  in 
the  geometric  form  of  the  body,  it  should  be  equal  to  the  cube  of  the 
increase  in  height;  this,  however,  is  3-37s=38.27,  a  number  well- 
nigh  twice  as  large  as  the  actual  increase. 

But  in  addition  to  these  changes,  which  are  largely  dependent 


472 


POST-NATAL    DEVELOPMENT 


upon  differences  in  the  supply  of  nutrition,  there  are  others  associ- 
ated with  alterations  in  the  general  metabolism  of  the  body.  Up 
to  adult  life  the  constructive  metabolism  or  anabolism  is  in  excess 
of  the  destructive  metabolism  or  katabolism,  but  the  amount  of  the 
excess  is  much  greater  during  the  earlier  periods  of  development 
and  gradually  diminishes  as  the  adult  condition  is  approached. 
That  this  is  true  during  intrauterine  life  is  shown  by  the  following 
figures,  compiled  by  Donaldson: 


Age  in  Weeks 

Weight  in  Grams 

Age  in  Weeks 

Weight  in  Grams 

o  (ovum) 

o . 0006 

24 

635 

4 

— 

28 

1,220 

8 

4 

32 

1,700 

12 

20 

36 

2,240 

16 

120 

40  (birth) 

3,250 

20 

285 

From  this  table  it  may  be  seen  that  the  embryo  of  eight  weeks 
is  six  thousand  six  hundred  and  sixty-seven  times  as  heavy  as  the 
ovum  from  which  it  started,  and  if  the  increase  of  growth  for  each 
of  the  succeeding  periods  of  four  weeks  be  represented  as  percent- 
ages, it  will  be  seen  that  the  rate  of  increase  undergoes  a  rapid 
diminution  after  the  sixteenth  week,  and  from  that  on  diminishes 
gradually  but  less  rapidly,  the  figures  being  as  follows : 


Periods  of  Weeks 

Percentage  Increase 

Periods  of  Weeks 

Percentage  Increase 

8-12 
12-16 
16-20 
20-24 

400 

500 

137 
123 

24-28 
28-32 
32-36 
36-40 

92 

39 
32 

45 

POST-NATAL    DEVELOPMENT 


473 


That  the  same  is  true  in  a  general  way  of  the  growth  after  birth 
may  be  seen  from  the  following  table,  representing  the  average 
weight  of  the  body  in  English  males  at  different  years  from  birth 
up  to  twenty-three  (Roberts),  and  also  the  percentage  rate  of 
increase. 


Year 

Number  of  Cases 

Weight  in 
Kilograms 

Percentage 
Increase 

o 

45i 

3-2 

i 

— 

(10.8) 

(238) 

2 

2 

14.7* 

(36)* 

3 

41 

15-4 

4.8* 

4 

102 

16.9 

9-7 

5 

*93 

18. 1 

7-i 

6 

224 

20.1 

11 

7 

246 

22  .6 

12.4 

8 

820 

24.9 

10.2 

9 

1,425 

27.4 

10 

IO 

1,464 

30.6 

"•5 

ii 

i,599 

32.6 

6-5 

12 

1,786 

34-9 

7 

x3 

2,443 

37-6 

7-7 

14 

2,952 

41.7 

10.9 

15 

3,"8 

46.6 

11. 7 

16 

2,235 

53-9 

15-7 

17 

2,496 

59-3 

10 

18 

2,15° 

62  .2 

4.9 

19 

i,438 

63-4 

1.9 

20 

851 

64.9 

2-5 

21 

738 

65-7 

1 .2 

22 

542 

67  .0 

1.9 

23 

55i 

67  .0 

0 

Certain  interesting  peculiarities  in  post-natal  growth  become 
apparent  from  an  examination  of  this  table.     For  while  there  is  a 

*  From  a  comparison  with  other  similar  tables  there  is  little  doubt  but  that  the 
weight  given  above  for  the  second  year  is  too  high  to  be  accepted  as  a  good  average 


474 


POST-NATAL   DEVELOPMENT 


general  diminution  in  the  rate  of  growth,  yet  there  are  marked 
irregularities,  the  most  noticeable  being  (i)  a  rather  marked  diminu- 
tion during  the  eleventh  and  twelfth  years,  followed  by  (2)  a  rapid 


Age 


LbsM 


14 


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s. 

Age 

Lbs/4 

"  12 

"  10 

•■  8 

"  6 

"   * 
'   Z 


Fig.  282. — Curves  Showing  the  Annual  Increase  in  Weight  in  (I)  Boys  and  (II) 

Girls. 
The  faint  line  represents  the  curve  from  British  statistics,  the  dotted  line  that  from 
American  (Bowditch),  and  the  heavy  line  the  average  of  the  two.     Before  the  sixth 
year  the  data  are  unreliable. — {Stephenson.) 

acceleration  which  reaches  its  maximum  at  about  the  sixteenth  year 
and  then  very  rapidly  diminishes.     These  irregularities  may  be  more 

Consequently  the  percentage  increase  for  the  second  year  is  too  high  and  that  for  the 
third  year  too  low. 

It  may  be  mentioned  that  the  weights  in  the  original  table  are  expressed  in  pounds 
avoirdupois  and  have  been  here  converted  into  kilograms,  and  further  the  figures  rep- 
resenting the  percentage  increase  have  been  added. 


POST-NATAL    DEVELOPMENT 


475 


clearly  seen  from  the  charts  on  page  474,  which  represent  the  curves 
obtained  by  plotting  the  annual  increase  of  weight  in  boys  (Chart  I) 
and  girls  (Chart  II).  The  diminution  and  acceleration  of  growth 
referred  to  above  are  clearly  observable  and  it  is  interesting  to  note 
that  they  occur  at  earlier  periods  in  girls  than  in  boys,  the  diminution 
occurring  in  girls  at  the  eighth  and  ninth  years  and  the  acceleration 
reaching  its  maximum  at  the  thirteenth  year. 

Considering,  now,  merely  the  general  diminution  in  the  rate 
of  growth  which  occurs  from  birth  to  adult  life,  it  becomes  interest- 
ing to  note  to  what  extent  the  organs  which  are  more  immediately 
associated  with  the  metabolic  activities  of  the  body  undergo  a  rela- 
tive reduction  in  weight.  The  most  important  of  these  organs  is 
undoubtedly  the  liver,  but  with  it  there  must  also  be  considered  the 
thyreoid  and  thymus  glands,  and  probably  the  suprarenal  bodies. 
In  all  these  organs  there  is  a  marked  diminution  in  size  as  compared 
with  the  weight  of  the  body,  as  will  be  seen  from  the  following  table 
(H.  Vierordt),  which  also  includes  data  regarding  other  organs  in 


ABSOLUTE  WEIGHT  IN  GRAMS. 

New-born  and  Adult. 


Liver 

Thy- 
reoid 

Thy- 
mus 

Suprarenal 
Bodies 

Spleen 

Heart 

Kidney 

„     .          Spmal 
Brain         _;     . 
Cord 

I4I-7 
1,819.0 

4-85 
33-8 

8.15 
26.9 

7-05 
7-4 

10.6 

163.0 

23.6 
300.6 

23-3 
3°5-9 

381.0 
1,430.9 

5-5 
39-iS 

PERCENTAGE  WEIGHT  OF  ENTIRE  BODY 

New-born  and  Adult. 

Liver 

Thy- 
reoid 

Thy- 
mus 

Suprarenal 
Bodies 

Spleen 

Heart 

Kidney 

Brain 

Spinal 
Cord 

4-57 
2-57 

0.16 

0.05 

0.26 
0.04 

0.23 

O.OI 

o-34 

0.25 

0.76 
0.46 

0-7S 
0.46 

12  .29 
2  .16 

0.18 
0.06 

476 


POST-NATAL    DEVELOPMENT 


which  a  marked  relative  diminution,  not  in  all  cases  readily  explain- 
able, occurs. 

Recent  observations  by  Hammar  render  necessary  some  modifica- 
tion of  the  figures  given  for  the  thymus  in  the  above  table.  He  finds  the 
average  weight  of  the  gland  at  birth  to  be  13.26  grms.,  and  that  the  weight 
increases  up  to  puberty,  averaging  37.52  grms.  between  the  ages  of  11  and 
15.  After  that  period  it  gradually  diminishes,  falling  to  16.27  grms- 
between  36  and  45,  and  to  6.0  grms.  between  66  and  75.  Expressed  in 
percentage  of  the  body  weight  this  gives  a  value  in  the  new-born  of  0.42 
and  in  an  individual  of  50  years  of  0.02,  a  difference  much  more  striking 
than  that  shown  in  Vierordt's  table. 

It  must  be  mentioned,  however,  that  the  gland  is  subject  to  much 
individual  variation,  being  largely  influenced  by  nutritive  conditions. 

The  remaining  organs,  not  included  in  the  tables  given  above, 
when  compared  with  the  weight  of  the  body,  either  show  an  increase 
or  remain  practically  the  same. 

ABSOLUTE  WEIGHT  IN  GRAMS. 
New-born  and  Adult. 


Skin  and  Sub- 
cutaneous Tissues 

Skeleton 

Stomach  and 
Musculature  ,    T 

Intestines 

Pancreas 

Lungs 

611.75 
11,765.0 

425-5 
ii,575-° 

776.5                   65 
28,732.0               1,364 

3-5 
97.6 

54-i 
994-9 

PERCENTAGE  OF  BODY-WEIGHT. 
New-born  and  Adult. 


Skin  and  Sub- 
cutaneous Tissues 

Skeleton 

Musculature 

Stomach  and 
Intestines 

Pancreas      Lungs 

19-73 
17.77 

13-7 
17.48 

25-05 
43-40 

2  . 1 
2  .06 

0. 11 
015 

i-75 
i-5o 

From  this  table  it  will  be  seen  that  the  greatest  increment  of 
weight  is  that  furnished  by  the  muscles,  the  percentage  weight  of 
which  is  one  and  three-fourths  times  as  great  in  the  adult  as  in  the 


POST-NATAL    DEVELOPMENT 


477 


child.  The  difference  does  not,  however,  depend  upon  the  differen- 
tiation of  additional  muscles;  there  are  just  as  many  muscles  in  the 
new-born  child  as  in  the  adult,  and  the  increase  is  due  merely  to  an 
enlargement  of  organs  already  present.  The  percentage  weight 
of  the  digestive  tract,  pancreas,  and  lungs  remains  practically  the 
same,  while  in  the  case  of  the  skeleton  there  is  an  appreciable  in- 
crease, and  in  that  of  the  skin  and  subcutaneous  tissue  a  slight 


. 


Fig.  283. — Longitudinal  Section  through  the  Sacrum  of  a  New-born  Female 

Child.— (Fehling.) 

diminution.  The  latter  is  readily  understood  when  it  is  remembered 
that  the  area  of  the  skin,  granting  that  the  geometric  form  of  the 
body  remains  the  same,  would  increase  as  the  square  of  the  length, 
while  the  mass  of  the  body  would  increase  as  the  cube,  and  hence 
in  comparing  weights  the  skin  might  be  expected  to  show  a  diminu- 
tion even  greater  than  that  shown  in  the  table. 


478 


POST-NATAL    DEVELOPMENT 


The  increase  in  the  weight  of  the  skeleton  is  due  to  a  certain 
extent  to  growth,  but  chiefly  to  a  completion  of  the  ossification  of 
the  cartilage  largely  present  at  birth.  A  comparison  of  the  weights 
of  this  system  of  organs  does  not,  therefore,  give  evidence  of  the 
many  changes  of  form  which  may  be  perceived  in  it  during  the  pe- 
riod under  consideration,  and  attention  may  be  drawn  to  some  of 
the  more  important  of  these  changes. 

In  the  spinal  column  one  of  the  most  noticeable  peculiarities 
observable  in  the  new-born  child  is  the  absence  of  the  curves  so 
characteristic  of  the  adult.  These  curves  are  due  partly  to  the  weight 
of  the  body,  transmitted  through  the  spinal  column  to  the  hip- 
joint  in  the  erect  position,  and  partly  to  the  action  of  the  muscles, 
and  it  is  not  until  the  erect  position  is  habitually  assumed  and  the 
musculature  gains  in  development  that  the  curvatures  become  pro- 
nounced. Even  the  curve  of  the  sacrum,  so  marked  in  the  adult, 
is  but  slight  in  the  new-born  child,  as  may  be  seen  from  Fig.  283, 
in  which  the  ventral  surfaces  of  the  first  and  second  sacral  verte- 
brae look  more  ventrally  than  posteriorly,  so  that  there  is  no  distinct 
promontory. 

But,  in  addition  to  the  appearance  of  the  curvatures,  other 
changes  also  occur  after  birth,  the  entire  column  becoming  much 
more  slender  and  the  proportions  of  the  lumbar  and  sacral  vertebrae 
becoming  quite  different,  as  may  be  seen  from  the  following  table 
(Aeby) : 


LENGTHS  OF  THE  VERTEBRAL  REGIONS  EXPRESSED  AS  PERCENT- 
AGES OF  THE  ENTIRE  COLUMN. 


Age 

Cervical 

Thoracic 

Lumbar 

New-born  child 

25.6 

23-3 
20.3 
19.7 
22  .1 

47-5 
46.7 

45-6 
47.2 
46.6 

26.8 

Male  2  years 

30.0 

Male  5  years 

34.2 

Male  1 1  years 

Male  adult 

33-i 
31.6 

POST-NATAL   DEVELOPMENT  479 

The  cervical  region  diminishes  in  length,  while  the  lumbar 
gains,  the  thoracic  remaining  approximately  the  same.  It  may  be 
noticed,  furthermore,  that  the  difference  between  the  two  variable 
regions  is  greater  during  youth  than  in  the  adult,  a  condition  pos- 
sibly associated  with  the  general  more  rapid  development  of  the 
lower  portion  of  the  body  made  necessary  by  its  imperfect  develop- 
ment during  fetal  life.  The  difference  is  due  to  changes  in  the 
vertebrae,  the  intervertebral  disks  retaining  approximately  the  same 
relative  thickness  throughout  the  period  under  consideration. 

The  form  of  the  thorax  also  alters,  for  whereas  in  the  adult  it  is 
barrel-shaped,  narrower  at  both  top  and  bottom  than  in  the  middle, 
in  the  new-born  child  it  is  rather  conical,  the  base  of  the  cone  being 
below.  The  difference  depends  upon  slight  differences  in  the  form 
and  articulations  of  the  ribs,  these  being  more  horizontal  in  the 
child  and  the  opening  of  the  thorax  directed  more  directly  upward 
than  in  the  adult. 

As  regards  the  skull,  the  processes  of  growth  are  very  compli- 
cated. Cranium  and  brain  react  on  one  another,  and  hence,  in 
harmony  with  the  relatively  enormous  size  of  the  brain  at  birth, 
the  cranial  cavity  has  a  relatively  greater  volume  in  the  child  than 
in  the  adult.  The  fact  that  the  entire  roof  and  a  considerable  part 
of  the  sides  of  the  skull  are  formed  of  membrane  bones  which,  at 
birth,  are  not  in  sutural  contact  with  one  another  throughout,  gives 
opportunity  for  considerable  modifications,  and,  furthermore,  the 
base  of  the  skull  at  the  early  stage  still  contains  a  considerable 
amount  of  unossified  cartilage.  Without  entering  into  minute  de- 
tails, it  may  be  stated  that  the  principal  general  changes  which  the 
skull  undergoes  in  its  post-natal  development  are  (i)  a  relative 
elongation  of  its  anterior  portion  and  (2)  an  increase  in  the  relative 
height  of  the  maxillae. 

If  a  line  be  drawn  between  the  central  points  of  the  occipital 
condyles,  it  will  divide  the  base  of  the  skull  into  two  portions,  which 
in  the  child's  skull  are  equal  in  length.  The  portion  of  the  skull  in 
front  of  a  similar  line  in  the  adult  skull  is  very  much  greater  than 
that  which  lies  behind,  the  proportion  between  the  two  parts  being 


480  POST-NATAL   DEVELOPMENT 

5:3,  against  3:3  in  the  child  (Froriep).  There  has,  therefore,  been 
a  decidedly  more  rapid  growth  of  the  anterior  portion  of  the  skull, 
a  growth  which  is  asssociated  with  a  corresponding  increase  in  the 
dorso-ventral  dimensions  of  the  maxillae.  These  bones,  indeed, 
play  a  very  important  part  in  determining  the  proportions  of  the 
skull  at  different  periods.  They  are  so  intimately  associated  with 
the  cranial  portions  of  the  skull  that  their  increase  necessitates  a 


Fig.  284. — Skull  of  a  New-born  Child  and  of  an  Adult  Man,  Drawn  as  of 
Approximately  the  Same  Size. — (Henke.) 


corresponding  increase  in  the  anterior  part  of  the  cranium,  and 
their  increase  in  this  direction  stands  in  relation  to  the  development 
of  the  teeth,  the  eight  teeth  which  are  developed  in  each  maxilla 
(including  the  premaxilla)  in  the  adult  requiring  a  longer  bone  than 
do  the  five  teeth  of  the  primary  dentition,  these  again  requiring  a 
greater  length  when  completely  developed  than  they  do  in  their 
immature  condition  in  the  new-born  child. 

But  far  more  striking  than  the  difference  just  described  is  that 
in  the  relative  height  of  the  cranial  and  facial  regions  (Fig.  284). 
It  has  been  estimated  that  the  volumes  of  the  two  portions  have  a 
ratio  of  8: 1  in  the  new-born  child,  4: 1  at  five  years  of  age,  and  2:1 
in  the  adult  skull  (Froriep) ,  and  these  differences  are  due  principally 
to  changes  in  the  vertical  dimensions  of  the  maxillae.  As  with  the 
increase  in  length,  the  increase  now  under  consideration  is,  to  a 


POST-NATAL   DEVELOPMENT  481 

ertain  extent  at  least,  associated  with  the  development  of  the  teeth, 
hese  structures  calling  into  existence  the  alveolar  processes  which 
,re  practically  wanting  in  the  child  at  birth.  But  a  more  important 
actor  is  the  development  of  the  maxillary  sinuses,  the  practically 
olid  bodies  of  the  maxillae  becoming  transformed  into  hollow  shells, 
rhese  cavities,  together  with  the  sinuses  of  the  sphenoid  and  frontal 
>ones,  which  are  also  post-natal  developments,  seem  to  stand  in 
elation  to  the  increase  in  length  of  the  anterior  portion  of  the  skull, 
erving  to  diminish  the  weight  of  the  portion  of  the  skull  in  front 
»f  the  occipital  condyles  and  so  relieving  the  muscles  of  the  neck  of  a 
onsiderable  strain  to  which  they  would  otherwise  be  subjected. 

These  changes  in  the  proportions  of  the  skull  have,  of  course, 
nuch  to  do  with  the  changes  in  the  general  proportions  of  the  face. 
3ut  the  changes  which  take  place  in  the  mandible  are  also  impor- 
ant  in  this  connection,  and  are  similar  to  those  of  the  maxillae  in 
leing  associated  with  the  development  of  the  teeth.  In  the  new- 
10m  child  the  horizontal  ramus  is  proportionately  shorter  than  in 
he  adult,  while  the  vertical  ramus  is  very  short  and  joins  the 
Lorizontal  one  at  an  obtuse  angle.  The  development  of  the  teeth 
if  the  primary  dentition,  and  later  of  the  three  molars,  necessitates 
,n  elongation  of  the  horizontal  ramus  equivalent  to  that  occurring 
n  the  maxillae,  and,  at  the  same  time,  the  separation  of  the  alveolar 
•orders  of  the  two  bones  requires  an  elongation  of  the  vertical  ramus 
f  the  condyle  is  to  preserve  its  contact  with  the  mandibular  fossa, 
,nd  this,  again,  demands  a  diminution  of  the  angle  at  which  the 
ami  join  if  the  teeth  of  the  two  jaws  are  to  be  in  proper  apposition. 

In  the  bones  of  the  appendicular  skeleton  secondary  epiphysial 
enters  play  an  important  part  in  the  ossification,  and  in  few  are 
hese  centers  developed  prior  to  birth,  while  the  union  of  the  epiphy- 
es  to  the  main  portions  of  the  bones  takes  place  only  toward  ma- 
urity.  The  dates  at  which  the  various  primary  and  secondary 
enters  appear,  and  the  time  at  which  they  unite,  may  be  seen  from 
he  following  table: 


31 


482 


POST-NATAL  DEVELOPMENT 
UPPER  EXTREMITY. 


Bone 

Appearance  of 

Appearance  of  Secondary 

Fusion  of 

Primary  Center 

Centers 

Centers 

Clavicle 

6th  week. 

(At  sternal  end)  17th  year. 

20th  year. 

Scapula. 

Body 

8th  week.          <. 

2  acromial  15th  year. 

2  on  vertical  border  16th  year. 

>  20th  year. 

Coracoid .... 

1  st  year. 

15  th  year. 

Head  1st  year. 

Great  tuberosity  3d  year. 

>  20th  year. 

Lesser  tuberosity  5th  year. 

J 

Humerus 

■jth  week. 

Inner  condyle  5th  year. 

1 8th  year. 

Capitellum  3d  year. 

1 

Trochlea  10th  year. 

[•  17  th  year. 

Outer  condyle  14th  year. 

J 

Ulna 

jth  week. 

Olecranon  10th  year. 

16th  year. 

Distal  epiphysis  4th  year. 

1 8th  year. 

Radius 

jth  week. 

Proximal  epiphysis  5th  year. 

17  th  year. 

Distal  epiphysis  2d  year. 

20th  year. 

Capita  turn 

1st  year. 

Hamatum 

2d  year. 

Triquetrum  . .  . 

3d  year. 

4th  year. 

Multangulum 

5th  year. 

majus. 

Navicular 

6th  year. 

Multangulum 

8th  year. 

minus. 

Pisiform 

12  th  year. 

Metacarpals . . . 

gth  week. 

3d  year. 

20th  year. 

Phalanges 

gth-nth  week. 

3d~5th  years. 

17  th-!  8th  years. 

The  dates  in  italics  are  before  birth. 


POST-NATAL    DEVELOPMENT 
LOWER  EXTREMITY. 


483 


Bone. 

Appearance 

of 

Appearance  of  Secondary                 Fusion  of 

Primary  Center 

Centers 

Centers 

gth  week. 

Crest  15th  year. 

Anterior  inferior  spine  15  th  year. 

V 

■  22d  year. 

Ischium 

4th  month. 

Tuberosity  15th  year. 

Pubis 

4th  month. 

Crest  1  Sth  year. 

Patella 

Cartilage  appears  at  4th  month,  ossification  in  3d  year. 

Head  1st  year. 

20  th  year. 

Femur 

■jth  week. 

J 

Great  trochanter  4th  year. 
Lesser  trochanter  13  th- 14th  year. 
Condyle  gth  month. 

19  th  year. 
1  Sth  year. 
2 1  st  year. 

Tibia 

jth  week. 

1 

Head  end  of  gth  month. 
Distal  end  2d  year. 

2ist-2  5thyear. 
1  Sth  year. 

Fibula 

Sth  week. 

/ 
\ 

Upper  epiphysis  5th  year. 
Lower  epiphysis  2d  year. 

21st  year. 
20th  year. 

Talus 

jth  month. 

Calcaneus 

6th  month. 

10th  year. 

1 6  th  year. 

Cuboid 

A  few  days 
after  birth. 

Navicular 

4th  year. 

Cuneiforms. 

1  st  year. 

Metatarsals 

gth  week. 

3d  year. 

20th  year. 

Phalanges 

gth-i2th  week 

4th-8th  years. 

I7th-i8th  years. 

The  dates  in  italics  are  before  birth. 

So  far  as  the  actual  changes  in  the  form  of  the  appendicular 
bones  are  concerned,  these  are  most  marked  in  the  case  of  the  lower 
limb.  The  ossa  innominata  alter  somewhat  in  their  proportions 
after  birth,  a  fact  which  may  conveniently  be  demonstrated  by  con- 
sidering the  changes  which  occur  in  the  proportions  of  the  pelvic 
diameters,  although  it  must  be  remembered  that  these  diameters 
are  greatly  influenced  by  the  development  of  the  sacral  curve. 
Taking  the  conjugate  diameter  of  the  pelvic  brim  as  a  unit  for  com- 
parison, the  antero-posterior  (dorso-ventral)  and  transverse  diame- 
ters of  the  child  and  adult  have  the  proportions  shown  in  the  table 
on  the  opposite  page  (Fehling). 


484 


POST-NATAL    DEVELOPMENT 


It  will  be  seen  from  this  that  the  general  form  of  the  pelvis  in 
the  new-born  child  is  that  of  a  cone,  gradually  diminishing  in  diam- 
eter from  the  brim  to  the  outlet,  a  condition  very  different  from 
what  obtains  in  the  adult.     Furthermore,  it  is  interesting  to  note 


Diameter. 


New-born 

Adult 

New-born 

Female. 

Female. 

Male. 

i  .00 

1 .00 

1 .00 

1. 19 

1 .292 

1 .20 

0.96 

1. 19 

0.91 

1 .01 

1. 151 

0.99 

0.91 

1.05 

0.78 

0.83 

i-i54 

0.84 

Adult 
Male. 


(Conjugata  vera . 
Transverse 

>,  f  Antero-posterior 

rt  1 

U   y  Transverse 

-^    (  Antero-posterior 

O      Transverse 


1.294 
1. 18 
1. 14 
1 .07 

1 -153 


that  sexual  differences  in  the  form  of  the  pelvis  are  clearly  distin- 
guishable at  birth;  indeed,  according  to  Fehling's  .observations, 
they  become  noticeable  during  the  fourth  month  of  intrauterine 
development. 

The  upper  epiphysis  of  the  femur  is  entirely  unossified  at  birth 
and  consists  of  a  cartilaginous  mass,  much  broader  than  the  rather 
slender  shaft  and  possessing  a  deep  notch  upon  its  upper  surface 
(Fig.  285).  This  notch  marks  off  the  great  trochanter  from  the 
head  of  the  bone,  and  at  this  stage  of  development  there  is  no  neck, 
the  head  being  practically  sessile.  As  development  proceeds  the 
inner  upper  portion  of  the  shaft  grows  more  rapidly  than  the  outer 
portion,  carrying  the  head  away  from  the  great  trochanter  and  form- 
ing the  neck  of  the  bone.  The  acetabulum  is  shallower  at  birth 
than  in  the  adult  and  cannot  contain  more  than  half  the  head  of 
the  femur;  consequently  the  articular  portion  of  the  head  is  much 
less  extensive  than  in  the  adult. 


POST-NATAL    DEVELOPMENT 


485 


It  is  a  well-known  fact  that  the  new-born  child  habitually  holds 
the  feet  with  the  soles  directed  toward  one  another,  a  position  only 
reached  in  the  adult  with  some  difficulty,  and  associated  with  this 
supination  or  inversion  there  is  a  pronounced  extension  of  the  foot 
(i.  e.,  flexion  upon  the  leg  as  usually  understood;  see  p.  102),  it  being 
difficult  to  flex  the  child's  foot  beyond  a  line  at  right  angles  with  the 
axis  of  the  leg.  These  conditions  are  due  apparently  to  the  ex- 
tensor and  tibialis  muscles  being  relatively  shorter  and  the  opposing 
muscles  relatively  longer  than  in  the  adult,  and  with  the  elongation 
or  shortening,  as  the  case  may  be,  of  the  muscles  on  the  assumption 


Fig.  2S5. — Longitudinal  Sections  of  the  Head  of  the  Femur  of  (.4)  New-born 

Child  and  (B)  a  Later  Stage  of  Development. 

ep,  Epiphysial  center  for  the  head;  h,  head;  /,  trochanter. — (Henke.) 

of  the  erect  position,  the  bones  in  the  neighborhood  of  the  ankle- 
joint  come  into  new  relations  to  one  another,  the  result  being  a  modi- 
fication of  the  form  of  the  articular  surfaces,  especially  of  the  talus 
(astragalus).  In  the  child  the  articular  cartilage  of  the  trochlear 
surface  of  this  bone  is  continued  onward  to  a  considerable  extent 
upon  the  neck  of  the  bone,  which  comes  into  contact  with  the  tibia 
in  the  extreme  extension  possible  in  the  child.  In  the  adult,  however, 
such  extreme  extension  being  impossible,  the  cartilage  upon  the  neck 
gradually  disappears.     The  supination  in  the  child  brings  the  talus 


486  LITERATURE 

in  close  contact  with  the  inner  surface  of  the  calcaneus  and  with 
the  sustentaculum  tali;  with  the  alteration  of  position  a  growth  of 
these  portions  of  the  calcaneus  occurs,  the  sustentaculum  becom- 
ing higher  and  broader,  and  so  becoming  an  obstacle  in  the  way  of 
supination  in  the  adult.  At  the  same  time  a  greater  extent  of  the 
outer  surface  of  the  talus  comes  into  contact  with  the  lateral 
malleolus,  with  the  result  that  the  articular  surface  is  considerably 
increased  on  that  portion  of  the  bone.  Marked  changes  in  the  form 
of  the  talo-navicular  articulation  also  occur,  but  their  consideration 
would  lead  somewhat  further  than  seems  desirable. 

LITERATURE. 

C.  Aeby:  "Die  Altersverschiedenheiten  der  menschlichen  Wirbelsaule."  Archiv  fur 

Anal,  und  Physiol.,  Anat.  Abth.,  1879. 
W.  Camerer:  "  Utersuchungen  iiber  Massenwachsthum  und  Langen  wachsthum  der 

Kinder,"  Jahrbuchfiir  Kinderheilkunde,  xxxvi,  1893. 
H.  H.  Donaldson:  "The  Growth  of  the  Brain,"  London,  1895. 
H.  Fehling:  "Die  Form  des  Beckens  beim  Fotus  und  Neugeborenen  und  ihre  Bezie- 

hung  zu  der  beim  Erwachsenen,"  Archiv  fur  Gynakol.,  x,  1876. 
H.  Friedenthal:  "  Das  Wachsthum  des  Korpergewichtes  des  Menschen  und  anderer 

Saugethiere  in  verschiedenen  Lebensaltern,"  Zeit.  allgem.  Physiol.,  ix,  1909. 
J.  A.  Hammar:  "Ueber  Gewicht,  Involution  und  Persistenz  der  Thymus  im  Post- 

fotalleben  des  Menschen,"  Archiv  fur  Anat.  und  Phys.,  Anat.  Abth.,  Supplement, 

1906. 
W.  Henke:  "  Anatomie  des  Kindersalters,"  Handbuch  der  Kinder krankheiten  (Cerhardt) , 

Tubingen,  1881. 
C.  Hennig:  "Das  kindliche  Becken,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1880. 
C.  Huter:  "Anatomische  Studien  an  den  Extremitatengelenken  Neugeborener  und 

Erwachsener,"  Archiv  fur  patholog.  Anat.  und  Physiol.,  xxv,  1862. 
W.  Stephenson:  "On  the  Relation  of  Weight  to  Height  and  the  Rate  of  Growth  in 

Man,"  TheLancet,  11,  1888. 
R.  Thoma:  "  Untersuchungen  iiber  die  Grosse  und  das  Gewicht  der  anatomischen 

Bestandtheile  des  menschlichen  Korpers,"  Leipzig,  1882. 
H.  Vierordt:  "Anatomische,  Physiologische  und  Physikalische  Daten  und  Tabellen," 

Jena,  1893. 
H.    Welcker:    "Untersuchungen    iiber    Wachsthum    und    Bau    des    menschlichen 

Schadels,"  Leipzig,  1862. 


NDEX 


After-birth,  137 
After-brain,  387 
Agger  nasi,  176 
Allan tois,  113,  361 
Alveolo-lingual  glands,  294 

groove,  290 
Amitotic  division,  7 
Amnion,  108,  109 
Amniotic  cavity,  54 
Amphiarthrosis,  188 
Amphiaster,  4 
Angioblast,  221 
Annulus  of  Vieussens,  233 
Anterior  commissure,  405 
Anthelix,  446 
Antitragus,  446 
Anus,  282 
Aortic  arches,  243 

bulb,  231 

septum,  236 
Archenteron,  48,  280 
Archoplasm  sphere,  4 
Arcuate  fibers,  391 
Areas  of  Langerhans,  313 
Arrectores  pilorum,  147 
Arteries,  240 

anterior  tibial,  253 

aorta,  244 

branchial,  242 

carotid,  243 

centralis  retinae,  459 

cceliac,  246 

common  iliac,  245 

epigastric,  250 

external  iliac,  247,  253 
maxillary,  243 

femoral,  254 

hyaloid,  448 

hypogastric,  247,  268 

inferior  mesenteric,  246 

innominate,  244 

intercostal,  245 

internal  mammary,  250 
maxillary,  242 
spermatic,  246 


Arteries,  interosseous,  251 

Ungual,  243 

lumbar,  245 

median,  251 

middle  sacral,  245 

peroneal,  254 

popliteal  253 

posterior  tibial,  255 

pulmonary,  243 

radial,  253 

renal,  246 

saphenous,  253 

sciatic,  253 

subclavian,  245 

superficial  radial,  251 

superior  intercostal,  248 
mesenteric,  246 
vesical,  247 

temporal,  242 

ulnar,  251 

umbilical,  116,  241,  247 

vertebral,  248 

vitelline,  119,  223 
Articular  capsule,  188 
Ary-epiglottic  folds,  335 
Arytenoid  cartilages,  336 
Aster,  4 
Atresia  of  duodenum,  306 

of  pupil,  453 
Atrial  septum,  233 
Atrio-ventricular  valves,  238 
Auerbach,  plexus  of,  420 
Auricle,  445 
Axis  cylinder,  378 


B 

Bartholin,  glands  of,  362 
Belly-stalk,  68,  114 
Bile  capillaries,  309 
Bladder,  359 
Blastoderm,  42 
Blastopore,  48,  54,  57 
Blastula,  39 
Blood,  224 

islands,  222 


487 


488 


INDEX 


Blood  platelets,  229 

vessels,  221 
Body  cavity,  48 
Bone,  development  of,  154 

growth  of,  157 
Bone-marrow,  156 
Bones: 

atlas,  162,  165 

axis,  165 

carpal,  184,  187,  482 

clavicle,  183,  482 

coccyx,  166 

conchae,  176 

epistropheus,  162,  165 

ethmoid,  174 

femur,  186,  483,  484 

fibula,  186,  483 

frontal,  178 

humerus,  184,  482 

hyoid,  182 

ilium,  186,  483 

incus,  179,  440 

innominate,  185,  483 

interparietal,  172 

ischium,  186,  483 

lachrymal,  178 

malleus,  179,  440 

mandible,  180 

maxilla,  179 

metacarpal,  185,  482 

metatarsal,  188,  482 

nasal,  178 

occipital,  170,  172 

palatine,  179 

parietal,  178 

patella,  186,  483 

periotic,  169,  176 

phalanges,  185,  188,  482,  483 

premaxilla,  179 

pubis,  186,  483 

radius,  184,  482 

ribs,  162,  165 

sacrum,  165 

scapula,  183,  482 

sphenoid,  173 

stapes,  441 

sternum,  166 

suprasternal,  166 

tarsal,  187,  483 

temporal,  176 

tibia,  186,  483 

turbinated,  175 

ulna,  184,  482 

vertebrae,  160,  164,  478 

vomer,  175 

zygomatic,  178 
Brachia  conjunctiva,  394 


Brain,  386,  475 
Branchial  arches,  90,  97 

clefts,  90 

epithelial  bodies,  294,  295 

fistula,  91 
Branchiomeres,  81 
Bronchi,  333 

Bucconasal  membrane,  283 
Bulbo-urethral  glands,  362 
Bulbo-vestibular  glands,  362 
Burdach,  fasciculus  of,  385 
Bursa  omentalis,  324 


Caecum,  301,  305 

Calcar,  403 

Canal  of  Cloquet,  463 

of  Gartner,  357 

of  Nuck,  365 

of  Petit,  463 
Canalized  fibrin,  128 
Capillaries,  224 
Cartilages  of  Santorini,  336 

of  Wrisberg,  336 
Caruncula  lacrimal  is,  468 
Cauda  equina,  384 
Caul,  112 
Cell,  1,  3 

division,  4 

theory,  1 
Centrosome,  4 
Cerebellum,  392 
Cerebral  aqueduct,  395 

convolutions,  402 

cortex,  407 

hemispheres,  398 

peduncles,  394 
Cheek  groove,  291 
Chin  ridge,  100 
Chondrocranium,  169,  172 
Chorda  canal,  57 

dorsalis,  75 

endoderm,  75 
Chorioid  coat,  449,  463 

plexus,  389,  397,  401 
Chorioidal  fissure  of  brain,  401 

of  eye,  448,  453 
Chorion,  67,118 

frondosum,  124 

laeve,  124 
Chorionic  villi,  123 
ChromafSne  organs,  370 
Chromatin,  3 
Cnromosomes,  4 

accessory,  15 

reduction  of,  14,  30 


INDEX 


489 


Ciliary  body,  454 

ganglion,  424 

muscle,  465 
Cisterna  chyli,  270 
Cleft  palate,  284 

sternum,  168 
Clitoris,  363 
Cloaca,  280,  360 
Cloacal  membrane,  287 
Cloquet,  canal  of,  463 
Coccygeal  ganglion,  275 
Ccelom,  48,  78 
Collateral  eminence,  404 
Colliculus  seminalis,  357 
Coloboma,  453 
Colon,  303 
Conjunctiva,  465 
Connective  tissues,  153 
Cornea,  449,  464 
Corniculate  cartilages,  336 
Corona  radiata,  21,  353 
Coronary  sinus,  232 
Corpora  mamillaria,  398 

quadrigemina,  395 
Corpus  albicans,  24 

callosum,  405 

luteum,  23 

striatum,  400 
Corti,  spiral  organ  of,  437 
Cowper,  glands  of,  362 
Cranial  nerves,  409 

sinuses,  255 
Cricoid  cartilage,  336 
Cuneiform  cartilages,  336 
Cutis  plate,  80 
Cytoplasm,  3 
Cyto-trophoblast,  122 


D 

Darwin's  tubercle,  446 
Decidua  basalis,  132 

capsularis,  121,  131 

reflexa,  121 

serotina,  132 

vera,  130 
Decidual  cells,  131,  137 
Dendrites,  379 
Dental  groove,  285 

papilla,  285 

shelf,  285 
Dentate  gyrus,  403 
Dermatome,  80 
Descent  of  ovary,  365 

of  testis,  366 
Diaphragm,  320 


Diarthrosis,  188 
Diencephalon,  387,  396 
Discus  proligerus,  19,  353 
Double  monsters,  46 
Duct  of  Santorini,  312 

of  Wrisberg,  312 
Ductus  arteriosus,  244,  268 

Botalli,  244 

choledochus,  307,  308 

cochlearis,  434 

Cuvieri,  257 

ejaculatorius,  355 

endolymphaticus,  433 

reuniens,  434 

venosus,  260 
Duodenum,  302,  303,  306 

E 

Ear,  431 

Ebner,  glands  of,  431 
Ectoderm,  48 
Embryo,  age  of,  102 

external  form,  86 

growth  of,  472 
Embryonic  disc,  54 
Embryotroph,  123 
Enamel  organ,  285 
Enchylema,  3 
Endocardium,  229 
Endoderm,  43 
Enveloping  layer,  42 
Ependymal  cells,  377 
Epiblast,  48 

Epibranchial  placodes,  417 
Epidermis,  141 
Epididymis,  354 
Epiglottis,  335 
Epiphyses,  156 
Epiphysis  cerebri,  396 
Epiploic  foramen,  324 
Episternal  cartilages,  166 
Epitrichium,  141 
Eponychium,  145 
Epoophoron,  356 
Erythrocytes,  225 
Erythroplastids,  226 
Eustachian  tube,  294,  440 

valve,  234 
Extrauterine  pregnancy,  22 
Eye,  446 
Eyelids,  465 


Fallopian  tubes,  357 
Fasciculus  communis,  414 


49° 


INDEX 


Fasciculus  of  Burdach,  385 

of  GoU,  385 

solitarius,  414 
Fenestra  cochleae,  440 

ovalis,  440 

rotunda,  440 

vestibuli,  440 
Fertilization  of  ovum,  31 
Fetal  circulation,  266 
Fibrinoid,  128 
Fifth  ventricle,  406 
Filum  terminale,  384 
Fimbria,  405 

_  ovarica,  357 
Foliate  papillae,  431 
Fontana,  spaces  of,  465 
Foramen  caecum,  296 

of  Winslow,  324 

ovale,  233,  240 
Fore-brain,  386 
Formatio  reticularis,  390 
Fornix,  405 
Frontal  sinuses,  176 
Funiculus  cuneatus,  385 

gracilis,  385 
Furcula,  294 


Gartner,  canals  of,  357 
Gall  bladder,  307,  308 
Ganglionated  cord,  422 
Gastral  mesoderm,  50,  62 
Gastrula,  48 
Geniculate  bodies,  398 
Genital  folds,  363 

ridge,  338,  349 

swelling,  363 

tubercle,  363 
Germ  cells,  7 

layers,  47,  60 

plasm,  8 
Giraldes,  organ  of,  354 
Glands  of  Bartholin,  362 

bulbo-urethral,  362 

bulbo-vestibular,  362 

of  Cowper,  362 

of  Ebner,  431 

Meibomian,  466 

of  MoU,  466 

salivary,  292 

tarsal,  466 
Goll,  fasciculus  of,  385 
Graafian  follicle,  19 
Great  omentum,  324 
Groove  of  Rosenmiiller,  295 


Gubernaculum  testis,  356 
Gynaecomastia,  151 


H 

Haematopoietic  organs,  225 
Haemolymph  nodes,  273 
Hairs,  146 
Hare  lip,  100,  179 
HassalPs  corpuscles,  298 
Haversian  canals,  158 
Head  cavities,  79 

process,  56,  69 
Heart,  229,  475 
Helix,  446 
Hensen's  node,  56 
Hermaphroditism,  365 
Hind-brain,  387 
Hippocampus,  402 
Hyaloid  canal,  463 
Hydatid  of  Morgagni,  355 

stalked,  359 
Hydramnios,  112 
Hymen,  357 
Hyperthelia,  151 
Hypertrichosis,  148 
Hypoblast,  48 
Hypochordal  bar,  161 
Hypophysis,  399 
Hypospadias,  365 
Hypothalamic  region,  398 


Implantation  of  ovum,  119 

Infracardial  bursa,  345 

Infundibulum,  399 

Inguinal  canal,  367 

Inner  cell  mass,  44 

Insula,  404 

Interarticular  cartilages,  189 

Intercarotid  ganglion,  373 

Intermediate  cell  mass,  77 

Interrenal  organs,  370 

Interventricular  foramen,  400 

Intervertebral  fibro-cartilage,  162 

Intestine,  301,  476 

Iris,  454 

Isthmus  cerebri,  387,  392 


J 

Jacobson,  organ  of,  429 

Joints,  188 

Jugular  lymph  sac,  286 


INDEX 


4QI 


K 

Karyokinesis,  7 
Karyoplasm,  3 
Kidney  (see  Metanephros) ,  343,  475 


Labia  majora,  363 
minora,  363 

Lachrymal  gland,  467 

Lamina  terminalis,  399 

Langerhans,  areas  of,  313 

Langhans  cells,  126 

Lanugo,  147 

Larynx,  334 

Lateral  thyreoids,  299 

Lens,  447,  450 

Lesser  omentum,  324 

Leukocytes,  227 

Ligaments: 

broad,  of  uterus,  349,  356 
coraco-humeral,  216 
coronary,  of  liver,  321 
falciform,  of  liver,  321 
fibular  lateral,  of  knee,  200 
flavan, 162 

inguinal,  349,  355,  357 
interspinous,  162 
of  the  ovary,  358 
pectinatum  iridis,  463 
round,  of  liver,  268 
round,  of  uterus,  358 
sacro-tuberous,  200 
spheno-mandibular,  180 
suspensory  of  lens,  462 

Limbs,  90,  100 

Lip-ridge,  100 

Lips,  284 

Liver,  306,  475 

Lungs,  331,  476  _ 

Luschka's  ganglion,  275 

Lymphatics,  268 

Lymph  nodes,  272 
sacs,  268,  270 

Lymphocytes,  227,  273 

M 

Mammary  gland,  148 
Mandibular  process,  92 
Mastoid  cells,  443 
Maturation  of  ovum,  28 
Maxillary  antrum,  176 

process,  92 
Meckel's  cartilage,  171,  179 

diverticulum,  113,  305 


Mediastina,  322 
Medulla  oblongata,  387 
Medullary  canal,  73,  88 

folds,  70,  72 

groove,  70 

sheath,  382 
Megacaryocytes,  228 
Meibomian  glands,  466 
Meissner,  plexus  of,  420 
Membrana  pupillaris,  453 

reuniens,  81 

tectoria,  437 
Membrane  bone,  154 
Menstruation,  26 
Mesamceboids,  222 
Mesencephalon,  387,  395 
Mesenchyme,  61 
Mesenteriole,  327 
Mesentery,  323 
Mesocardium,  316 
Mesocolon,  326 
Mesoderm,  48 

somatic,  78 

splanchnic,  78 

ventral,  77 
Mesodermic  somites,  72,  76 
Mesogastrium,  324 
Mesonephros,  341 
Mesorchium,  367 
Mesothelium,  61 
Metamere,  83 
Metanephros,  343 
Metencephalon,  387,  392 
Mid-brain,  387 
Middle  ear,  440 
Milk  ridge,  148 
Mitosis,  7 
Moll,  glands  of,  466 
Montgomery's  glands,  150 
Morgagni,  hydatid  of,  355 
Morula,  43 
Mouth  cavity,  283 
Mtillerian  duct,  347 
Muscle  plates,  80 
Muscles: 

arrectores  pilorum,  147 

biceps  femoris,  216 

branchiomeric,  206 

chondroglossus,  208 

ciliary,  465 

coccygeus,  204 

constrictor  of  pharynx,  208,  299 

cranial,  205 

curvator  coccygis,  204 

depressors  of  hyoid,  202 

digastric,  206 

dilatator  iridis,  455 


492 


INDEX 


Muscles,  dorsal,  200 

eye,  205 

facial,  206 

gastrocnemius,  215,  219 

geniohyoid,  202 

genioglossus,  202 

glosso-palatinus,  208 

hyoglossus,  202 

hyposkeletal,  202 

intercostal,  202 

laryngeal,  208 

latissimus  dorsi,  198 

levator  ani,  204 

limb,  210 

longus  capitis,  202 
colli,  202 

lumbrical,  218 

masseter,  206 

mylohyoid,  206 

obliqui  abdominis,  202 

occipito-fron talis,  198,  206 

omohyoid,  198 

pectorals,  216 

perineal,  204 

peroneus  longus,  216 

platysma,  206 

pronator  quadratus,  216 

psoas,  202 

pterygoids,  206 

pyramidalis,  202 

rectus  abdominis,  199,  202 

sacro-spinalis,  199,  204 

scaleni,  202 

serrati  posteriores,  199 

serratus  anterior,  199 

skeletal,  197 

soleus,  215,  219 

sphincter  ani,  204 
cloacae,  205 
iridis,  455 

stapedius,  206,  441 

sternohyoid,  198 

sternomastoid,  198,  202,  208 

styloglossus,  202 

stylohyoid,  206 

stylopharyngeus,  208,  299 

temporal,  206 

tensor  tympani,  206,  440 
veli  palati,  206 

transversus  abdominis,  202 
thoracis,  202 

trapezius,  198,  202,  208 
Muscle  tissue,  193 
Myelencephalon,  387,  389 
Myelin,  382 
Myelocytes,  227 
Myoblasts,  195 


Myocardium,  229 
Myotome,  80,  198 


N 


Nails,  144 
Nape  bend,  90 
Nasal  pit,  99 

process,  99 
Naso-lachrymal  duct,  467 
Nephrogenic  cord,  342 
Nephrostome,  340 
Nephrotome,  80 
Nerve  components,  410,  413 

roots,  380 
Nerves: 

auditory,  415 

cranial,  409 

hypoglossal,  412 

olfactory,  428 

optic,  458 

recurrent,  337 

spinal,  408 

accessory,  416 

splanchnic,  424 
Nerve  tissue,  377 
Neural  crest,  380 
Neurenteric  canal,  58,  69,  73 
Neuroblasts,  378 
Neuroglia  cells,  378,  379 
Neuromeres,  418 
Neurone  theory,  382 
Nitabuch's  stria,  135 
Non-sexual  reproduction,  8 
Normoblasts,  226 
Notochord,  74 
Nuck,  canal  of,  365 
Nucleoli,  4 
Nucleus,  3 

O 

(Esophagus,  299 
OEstrus,  27 
Odontoblasts,  287 
Olfactory  lobes,  406 

organ,  428 
Olivary  body,  390 
Omentum,  324 
Oocyte,  29 
Optic  cup,  448,  453 

recess,  399 
Oral  fossa,  88,  99,  280 
Organ  of  Giraldes,  354 

of  Jacobson,  429 

of  Rosenmuller,  356 


INDEX 


493 


Organs,  2 

of  taste,  430 

of  Zuckerkandl,  374 

Osteoblasts,  154 

Osteoclasts,  158 

Otocyst,  432 

Otic  ganglion,  424 

Ovary,  352 

descent  of,  365 

Ovulation,  21,  26 

Ovum,  19 

fertilization  of,  31 
implantation  of,  119 
maturation  of,  28 
segmentation  of,  38 


Palate,  283 
Pancreas,  311,  476 
Paradidymis,  354 
Paraphysis,  397 
Parathymus,  299 
Parathyreoid  bodies,  297 
Paroophoron,  356 
Parotid  gland,  292 
Parovarium,  356 
Parthenogenesis,  8 
Penis,  364 

Pericardial  cavity,  317,  318 
Perineal  body,  362 
Perionyx,  145 
Periosteum,  155 
Periotic  capsule,  169,  176 
Peritoneum,  323 
Petit,  canal  of,  463 
Pfliiger's  cords,  352 
Pharyngeal  bursa,  294 

membrane,  280 

tonsil,  294 
Pharynx,  294 

Pharyngo-palatine  arches,  283 
Pineal  body,  396 
Pinna,  445 
Pituitary  body,  399 
Placenta,  133,  137 

accessory,  126 

embryotrophic,  123 

haematrophic,  123 

prsevia,  133 
Placentar  infarcts,  135 
Plasmodi-trophoblast,  122 
Plasmodium,  122 
Pleurae,  322 

Pleuro-peritoneal  cavity,  78,  320 
Plica  semilunaris,  466 


Polar  globules,  30 
Polycaryocytes,  228 
Polymastia,  151 
Polyspermy,  34 
Pons,  392 

flexure,  389 
Post- anal  gut,  281 
Post-natal  development,  470 
Posterior  lymph  sac,  270 
Precaudal  recess,  281 
Precoracoid,  189 
Prepuce,  364 
Primitive  groove,  56,  69 

streak,  50,  69 
Processus  globularis,  99 
Pronephric  duct,  339 
Pronephros,  339 
Pronuclei,  31 
Procestrum,  27 
Prostate  gland,  362 
Prostomial  mesoderm,  50,  58 
Protoplasm,  2 
Proto vertebrae,  77 


R 


Rathke's  pouch,  285,  399 

Rauber's  covering  layer,  44 

Rectum,  281 

Red  nucleus,  395 

Reduction  of  chromosomes,  14,  30 

Restiform  body,  391 

Rete  cords,  349 

ovarii,  354 

testis,  352 
Retina,  455 

Retroperitoneal  lymph  sac,  270 
Rhinencephalon,  407 
Rosenmuller,  groove  of,  295 

organ  of,  356 


Sacculus,  434 
Sacral  bend,  90 
Salivary  glands,  291 
Santorini,  cartilages  of,  336 

duct  of,  312 
Sarcode,  1 
Scala  tympani,  440 

vestibuli,  439 
Sclerotic  coat,  449,  463 
Sclerotome,  80 
Scrotum,  364 


494 


INDEX 


Sebaceous  glands,  147 
Segmentation  of  ovum,  38 
Semicircular  ducts,  433 
Semilunar  valves,  239 
Seminiferous  tubules,  352 
Septum  pellucidum,  406 

primum,  233 

secundum,  233 

spurium,  232 

transversum,  318,  320,  323 
Sertoli  cell,  14 
Sex  cells,  349 

cords,  349 
Sexual  reproduction,  8 
Sinusoid,  223 
Sinus,  coronary,  232 

pocularis,  355 

praecervicalis,  97 

terminalis,  222 

venosus,  230 
Situs  inversus  viscerum,  46 
Skin,  141,  476 
Skull,  168,  479 
Socia  parotidis,  291 
Solitary  fasciculus,  390 
Somatic  cells,  7 
Spaces  of  Fontana,  465 
Spermatic  cord,  367 
Spermatid,  14 
Spermatocyte,  14 
Spermatogenesis,  13 
Spermatogonia,  14 
Spermatozoon,  11 
Sphenoidal  cells,  176 
Spheno-palatine  ganglion,  424 
Spinal  cord,  383,  475 

nerves,  408 
Spiral  organ  of  Corri,  437 
Spleen,  274,  475 
Stomach,  301 
Sublingual  ganglion,  424 

gland,  293 
Submaxillary  ganglion,  424 

gland,  292 
Substance  islands,  222 
Sudoriparous  glands,  148 
Sulcus  Monroi,  396 
Superfetation,  36 
Suprabranchial  placodes,  417 
Suprarenal  bodies,  370,  475 

accessory,  372 
Supratonsillar- fossa,  295 
Suture,  188 

Sympathetic  nervous  system,  418 
Synchondrosis,  188 
Syncytium,  122 
Systems,  2 


Tail  filament,  94 
Tarsal  glands,  466 
Taste,  organs  of,  430 
Teeth,  285 
Tegmentum,  394 
Telencephalon,  386,  398 
Testis,  350 

descent  of,  366 
Thalami,  397 
Thebesian  valve,  234 
Thoracic  duct,  271 
Thymus  gland,  297,  476 
Thyreoid  cartilage,  335 

gland,  296,  475 
Thyreo-glossal  duct,  296 
Tissues,  2 
Tongue,  289 
Tonsils,  295 
Touch,  organs  of,  430 
Trachea,  334 
Tragus,  446 
Trophoblast,  55 
Tuba  auditiva,  440 
Tubae  uterinse,  357 
Tuber  cinereum,  398 
Tuberculum  impar,  289 
Tunica  vaginalis  testis,  367 

vasculosa  lentis,  452 
Tween-brain,  387 
Twin-development,  46 
Tympanic  cavity,  442 

membrane,  443 


U 

Ultimo-branchial  bodies,  299 
Umbilical  cord,  92,  116 
Umbilicus,  86 
Urachus,  115,  361 
Ureter,  344 
Urethra,  361 
Urogenital  sinus,  360 
Uterovaginal  canal,  349 
Uterus,  357,  359 

masculinus,  355 
Utriculus,  434 

prostaticus,  355 


V 

Vagina,  357 
Vaginal  process,  365 
Vallate  papillae,  430 


INDEX 


495 


Vas  deferens,  355 
Veins: 

anterior  cardinal,  255 
tibial,  265 

ascending  lumbar,  264 

azygos,  264 

basilic,  265 

cephalic,  265 

emissary,  259 

external  jugular,  258 

hemiazygos,  264 

hepatic,  262 

inferior  vena  cava,  263 

innominate,  258 

internal  jugular,  255 

jugulo-cephalic,  265    ■ 

limb,  265 

long  saphenous,  265 

portal,  261 

posterior  cardinal,  255 

primary  fibular,  265 
ulnar,  265 

pulmonary,  265 

renal,  263 

subcardinal,  262 

superior  vena  cava,  258 

supracardinal,  263 

suprarenal,  263 

umbilical,  116,  260 

vitelline,  223,  259 
Velum,  anterior,  394 

interpositum,  397 

marginal,  378 


Velum,  posterior,  389 
Ventricular  septum,  236 
Vermiform  appendix,  305 
Vernix  caseosa,  112,  147 
Vertex  bend,  86 
Vesicula  seminalis,  355 
Vieussens,  annulus  of,  233 
Villi,  chorionic,  123 
intestinal,  305 
Vitreous  humor,  449,  461 
Vulva,  363 

W 

Wharton's  jelly,  118 
Winslow,  foramen  of,  324 
Wirsung,  duct  of,  312 
Witch  milk,  151 
Wolffian  body,  341,  354 

duct,  339,  354 

ridge,  338 
Wrisberg,  cartilage  of,  336 


Yolk  sac,  86,  112 

stalk,  86,  90,  112 


Zona  pellucida,  21 
Zuckerkandl,  organ  of,  374 


BOOKS  FOR  STUDENTS  OF  BIOLOGY. 

KINGSLEY.  Comparative  Anatomy  of  Vertebrates.  A  text-book 
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Biology,  James  Milliken  University,  Decatur,  Illinois.  Second  Edition, 
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Elementary  Zoology.  A  Text-book  for  Secondary  Educational 
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GREEN.  Vegetable  Physiology,  An  Introduction  to.  By  J.  Reynolds 
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JOHNSTON.  Nervous  System  of  Vertebrates.  By  John  Black 
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MARSHALL.  Microbiology.  A  Text-book  of  Microorganisms,  General 
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VINAL.     A  Guide  for  Laboratory  and  Field  Studies  in  Botany.     By 

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STOHR.  Text -book  of  Histology.  Arranged  upon  an  Embryological 
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CRARY.     Field  Zoology,  Insects  and  Their  Near  Relatives  and  Birds. 

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PATTEN.  The  Evolution  of  the  Vertebrates  and  Their  Kin.  By 
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